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BUREAU OF MINES 
INFORMATION CIRCULAR/1988 



Recent Developments in Metal and 
Nonmetal Mine Fire Protection 

Proceedings: Bureau of Mines Technology 
Transfer Seminars, Denver, CO, October 18-19; 
Detroit, Ml, October 20-21; Las Vegas, NV, 
November 1-2; and Spokane, WA, November 
3-4, 1988 



By Staff, Bureau of Mines 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 



[ / lirjXtJ &%&*, BMJUmtf \hw) 



Recent Developments in Metal and 
Nonmetal Mine Fire Protection 

Proceedings: Bureau of Mines Technology 
Transfer Seminars, Denver, CO, October 18-19; 
Detroit, Ml, October 20-21; Las Vegas, NV, 
November 1-2; and Spokane, WA, November 
3-4, 1988 

By Staff, Bureau of Mines 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
T S Ary, Director 







A 



0* 



<\~z 



0(0 



Library of Congress Cataloging in Publication Data: 



Bureau of Mines Technology Transfer Seminars (1988 : Denver, 
Colo., etc. 

Recent developments in metal and nonmetal mine fire protections. 

(Bureau of Mines Information circular; 9206) 

Includes bibliographical references. 

Supt. of Docs, no.: I 28.27:9206. 

1. Mine fires-Prevention and control-Congresses. I. United States. Bureau of 
Mines. II. Title. III. Series: Information circular (United States. Bureau of Mines); 
9206. 



TN295.U4 



[TN315] 



622 s [622'.8] 



88-600299 



PREFACE 

In October and November 1988, the Bureau of Mines held technology transfer seminars on 
metal-nonmetal mine fire protection at Denver, CO, Detroit, MI, Las Vegas, NV, and Spokane, 
WA. The papers presented at those seminars are contained in this Information Circular. The papers 
highlight Bureau research to improve mine fire protection. Areas addressed by this research, and 
published in this volume, include fire detection and instrumentation, fire warning, fire suppres- 
sion, diesel equipment, spontaneous combustion, and toxicity analysis of combustion products. Cer- 
tain of the findings are also applicable to underground coal mining and to surface mining operations. 

The technology transfer seminar used as a forum for the transfer of this research is one of 
the many mechanisms used by the Bureau of Mines in its efforts to move research developments, 
technology, and information resulting from its programs into industrial practice and use. To learn 
more about the Bureau's technology transfer program and how it can be useful to you, please write 
or telephone: 

Bureau of Mines 

Office of Technology Transfer 

2401 E Street, NW. 

Washington, DC 20241 

202-634-1224 



Ill 



CONTENTS 

Page 

Preface i 

Abstract 1 

Introduction 2 

Statistical Analysis of Metal and Nonmetal Mine Fire Incidents in the United States From 1950 to 1984, 

by Shail J. Butani and William H. Pomroy 3 

Computer Models of Underground Mine Ventilation and Fires, by Rudolf E. Greuer 6 

Mine Fire Detection Systems: A Primer, by Charles D. Litton 15 

Fire Detection Systems for Noncoal Underground Mines, by W.H. Pomroy 21 

Diesel-Discriminating Fire Sensor, by Charles D. Litton 28 

Computer-Aided Mine Fire Sensor Data Interpretation in Real Time, by L.W. Laage, 

W.H. Pomroy, and A.M. Bartholomew 33 

Reliability of Underground Mine Fire Detection and Suppression Systems, by Steven G Grannes 42 

Diesel Exhaust Conditioning Systems for Fire and Explosion Control in Gassy Mines, by Kenneth L. Bickel 49 

Spontaneous Combustion Susceptibility of Sulfide Minerals, by G.W. Reimers and W.H. Pomroy 54 

Emission Products From Wood Crib and Transformer Fluid Fires, by Margaret R. Egan 61 

Utilization of Smoke Properties for Predicting Smoke Toxicity, by Maria I. De Rosa and Charles D. Litton 72 

Electromagnetic Fire Warning System for Underground Mines, by Kenneth E. Hjelmstad and William H. Pomroy 78 



LIST OF UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


A 


angstrom 


km/h 


kilometer per hour 


A 


ampere 


kW 


kilowatt 


A/m 


ampere per meter 


lb 


pound 


acre-ft 


acre-foot 


L/min 


liter per minute 


Btu/min 


British thermal unit per minute 


m 


meter 


cm 


centimeter 


mCi 


millicurie 


cm 2 /p 


square centimeter per particle 


mg 


milligram 


cm 2 /(p«g) 


square centimeter per particle per gram 


mg/m 3 


milligram per cubic meter 


cm 3 /min 


cubic centimeter per minute 


mho/m 


mho per meter 


°C 


degree Celsius 


min 


minute 


°C/min 


degree Celsius per minute 


mm 


millimeter 


dB/m 


decibel per meter 


m/s 


meter per second 


ft 


foot 


m 2 


square meter 


ft/min 


foot per minute 


m 3 


cubic meter 


°F 


degree Fahrenheit 


m 3 /s 


cubic meter per second 


g 


gram 


/iCi 


microcurie 


gal 


gallon 


Mg/m 3 


microgram per cubic meter 


g/cm 3 


gram per cubic centimeter 


^m 


micrometer 


g/g 


gram per gram 


p/cm 3 


particle per cubic meter 


g/kJ 


gram per kilojoule 


pet 


percent 


g/(m 3 «ppm) 


gram per cubic meter per part per 


p/kJ 


particle per kilojoule 




million 


psi 


pound per square inch 


g/min 


gram per minute 


ppm 


part per million 


g/s 


gram per second 


ppm/g 


part per million per gram 


h 


hour 


ppm/min 


part per million per minute 


hp 


horsepower 


s 


second 


Hz 


hertz 


St 


short ton 


in 


inch 


V 


volt 


kg 


kilogram 


V dc 


volt, direct current 


kHz 


kilohertz 


W 


watt 


kJ/g 


kilojoule per gram 


yr 


year 



RECENT DEVELOPMENTS IN METAL AND NONMETAL 
MINE FIRE PROTECTION 

Proceedings: Bureau of Mines Technology Transfer Seminars, 

Denver, CO, October 18-19; Detroit, Ml, October 20-21; 

Las Vegas, NV, November 1-2; and Spokane, WA, November 3-4, 1988 



By Staff, Bureau of Mines 



ABSTRACT 

Great strides have been made in recent years to reduce the disaster potential of underground 
mine fires. However, mines can still be caught unprepared for a fire emergency. Fires can grow 
too large before they are detected, warning systems can be too slow and uncertain to reliably signal 
the danger, and suppression systems can be inadequate to extinguish the flames. New mining systems 
and equipment may create unanticipated fire hazards, and new materials may generate highly toxic 
combustion products. This report contains papers that summarize recent significant developments 
from the Bureau of Mines mine fire protection research program relating to these problems. Cer- 
tain of these findings are also applicable to surface mining operations. The papers fall into the 
general categories of fire detection and instrumentation, fire warning, fire suppression, diesel equip- 
ment, spontaneous combustion, and toxicity analysis of combustion products. 



INTRODUCTION 



No peril is more feared by miners than an underground fire. 
Fresh air is limited and the workings can rapidly fill with choking 
smoke and fire gases. Careful attention to fire prevention is the first 
priority . Special precautions must be taken to limit ignition and fuel 
sources underground. In the unlikely event that a fire does occur, 
quick action is essential — miners must be warned and evacuated, 
and fire-fighting operations must be initiated. 

Great strides have been made in recent years to reduce the 
disaster potential of underground fires, as evidenced by the steady 
decline in the number of fire incidents and the number of related 
injuries and fatalities recorded over the past four decades. However, 
mines can still be caught unprepared for a fire emergency. 

Often, fires grow too large before they are detected, warning 
systems are too slow and uncertain to reliably signal the danger, 
and suppression systems are lacking or are inadequate to extinguish 
the flames. In addition, new mining systems and equipment may 
create unanticipated fire hazards and new materials may generate 
highly toxic combustion products. The result is loss of life, damage 
to mining facilities and equipment, and loss of valuable minerals 
resources. 

Since 1978, about 10 underground metal and nonmetal mine 

fires have been reported annually to the Mine Safety and Health 

Administration (MSHA). Each year another estimated 100 to 150 

fires occur that are legally "nonreportable" because they last less 

. than 30 min or do not cause an injury. With nearly 20,000 mine 



accidents occurring annually in the United States, mine fires are, 
by comparison, quite rare events. But unlike most mine accidents, 
every fire has the potential to develop into a mine disaster. It is 
also significant to note that the fire incidence rate, or the number 
of fires per worker-hour of exposure, is actually higher in 
underground mines than for aboveground industrial occupancies. 

Since its creation in 1910, the Bureau of Mines has been com- 
mitted to developing the necessary mining and safety technology 
to reduce or eliminate fire hazards in underground mines. Indeed, 
much of the improvement in mine fire safety performance during 
this century can be traced to pioneering research by the Bureau. 
This tradition continues to the present with a vigorous program of 
basic and applied research aimed at devising practical solutions to 
the industry's most pressing mine fire problems. This effort 
represents a mutually beneficial partnership of the Bureau, MSHA, 
and the mining community. 

The papers contained in this Information Circular summarize 
recent significant developments from the Bureau's mine fire pro- 
tection research program that are relevant to underground metal 
and nonmetal mining; certain of these findings are also applicable 
to underground coal mining and to surface mining operations. The 
papers fall into the general categories of fire detection and instrumen- 
tation, fire warning, fire suppression, diesel equipment, spontaneous 
combustion, and toxicity analysis of combustion products. 



STATISTICAL ANALYSIS OF METAL AND NONMETAL MINE FIRE INCIDENTS 
IN THE UNITED STATES FROM 1950 TO 1984 



By Shail J. Butani 1 and William H. Pomroy 2 



ABSTRACT 

This paper presents the results of a Bureau of Mines analysis of Mine Safety and Health Ad- 
ministration (MSHA) mine fire reports for the 1950-84 period. The analysis also includes non- 
reportable fires (less than 30-min duration and no injury) to show the magnitude of the problem. 
The most frequent ignition sources, burning substances, equipment types, fire locations, and suc- 
cessful extinguishing agents are discussed. Fire incidence rates in both surface and underground 
metal and nonmetal mines are not decreasing. New mining technology introduces new fire hazards 
into the workplace; fire safety emphasis must focus on newly emerging mining technologies in 
order to reduce incidence rates. 



INTRODUCTION 



In support of the Bureau's program of mine fire protection 
research, two separate baseline studies of fire incidents have been 
conducted. The first study addressed coal mine fires 3 and the sec- 
ond addressed metal and nonmetal mine fires. 4 Together, these two 
reports provide a comprehensive factual summary of the mining 
industry's fire experience. 

The purpose of this paper is to summarize the most significant 
findings of the second study, which analyzed separately all official 
MSHA mine fire reports prepared during the 1950-84 period, plus 
accounts of selected nonreportable fires (i.e., less than 30 min and 
no injury) and mine safety director hazard opinion data. Nonreport- 
able fires were included to show the true magnitude of the fire 
problem. 

Because metal and nonmetal mines have been legally required 
to report fires to MSHA only since 1968, MSHA files prior to that 
date are incomplete. Although some fires were reported prior to 



'Mathematical statistician (now with Bureau of Labor Statistics, Washington. DC) 

2 Group supervisor. 
Twin Cities Research Center, Bureau of Mines. Minneapolis. MN. 

'McDonald. L. B., and W. H. Pomroy. A Statistical Analysis of Coal Mine Fire 
Incidents in the United States From 1950 to 1977. BuMines IC 8830, 1980, 42 pp. 

4 Butani, S. J., and W. H. Pomroy. A Statistical Analysis of Metal and Nonmetal 
Mine Fire Incidents in the United States From 1950 to 1984. BuMines IC 9132, 1987. 
41 pp. 



1968, doubtless a great many were not. Also, the reporting regula- 
tions that took effect in 1968 specify that only fires lasting 30 min 
or longer or involving an injury need to be reported. However, fires 
lasting less than 30 min and involving no injury pose a significant 
hazard, and much can be learned from such incidents. MSHA fire 
reports are thus limited in scope by MSHA's legal authority. To 
provide a more comprehensive data base, it was necessary to gather 
and analyze mine company records of nonreportable fires. Opin- 
ion data from mine safety directors were collected and separately 
analyzed in an effort to broaden the discussion and to characterize 
and rank mine fire hazards in general. 

Because mine fires are relatively rare events, it is desirable 
not only to analyze the fires themselves, but also the near-misses 
(which occur much more frequently) and the unsafe conditions that 
could give rise to future fires. The opinion data give the necessary 
insight into near-misses and unsafe conditions. 

Where possible, both reported and nonreportable fires were 
analyzed by time trends (1950-67, 1968-77, 1978-84), ore type, 
ignition source, burning substance, location in mine, equipment in- 
volved, means of detection, duration, number of injuries, number 
of fatalities, mining method, and successful extinguishing agent. 
The analysis was performed separately for underground fire inci- 
dents and for surface fire incidents occurring at surface mines or 
at surface locations of an underground mine. 



RESULTS 



Major findings of the study appear in tables 1 and 2. The most 
frequent ignition sources, burning substances, equipment types, 
locations, and successful extinguishing agents of reported and non- 
reportable fires are discussed in the following sections. 



IGNITION SOURCE 

The most frequent ignition source in underground mine fires 
is electricity. This is true for both reported and nonreportable fires 



Table 1.— Major Study Findings of Reported Fires 



Category 



1950-77 



1 978-84 



Overall 



UNDERGROUND 



Ore type Iron, copper Lead-zinc, salt Iron, copper, lead-zinc, salt. 

Ignition source' Electrical, welding Electrical, engine heat Electrical, welding, engine heat. 



Burning substance 1 Timber, insulation, combustible 

liquids. 

Location 1 Haulageway-drift, shaft-raise- 
winze. 



Combustible liquids, timber, 
insulation. 

Haulage-drift, shaft-raise- 
winze. 



Timber, combustible liquids, 
insulation. 

Haulageway-drift, shaft-raise- 
winze. 



Equipment involved 1 Mobile, electrical Mobile, electrical Mobile, electrical. 



Means of detection 1 Workers (not immediate), operators- Operators-workers (immediate), 

workers (immediate). workers (not immediate). 



Operators-workers (immediate), 
workers (not immediate). 



Duration h. . 24+, 1+ to 4 1+ to 4, 24+ 1+ to 4, 24+. 

Successful extinguishing agent 1 ... Water-dry chemical Water-dry chemical Water-dry chemical. 



SURFACE 



Ore type Iron, crushed limestone Crushed limestone, iron Crushed limestone, iron. 

Ignition source 1 Electrical, welding, engine heat. . . . Engine heat, welding, electrical. . . . Engine heat, electrical, welding. 



Burning substance 1 Combustible liquids, construction 

material. 



Combustible liquids, construction 
material. 



Combustible liquids, construction 
material. 



Location 1 Surface building Surface, surface building Surface building, surface. 

Equipment involved 1 Mobile, electrical Mobile, conveyor Mobile, conveyor, electrical. 



Means of detection 1 Operators-workers (immediate), 

workers (not immediate). 



Operators-workers (immediate), 
workers (not immediate). 



Operators-workers (immediate), 
workers (not immediate). 



Duration h . 

Successful extinguishing agent 1 . . . 



to 0.5 to 0.5 to 0.5. 

Water, burned out Water, dry chemical Water, burned out. 



1 Factors listed in sequence of significance. 



Table 2.— Major study findings of nonreportable fires 

(Factors listed in sequence of significance) 



Category 



Underground 



Surface 



NONREPORTABLE FIRES 



Ignition source 

Burning substance 

Location 

Equipment involved 

Successful extinguishing agent. 



Electrical, welding, friction Welding, electrical, engine heat. 

Combustible liquids, wiring insulation, timber .... Combustible liquids, insulation. 

Haulageway-drift, substation Surface, surface building. 

Mobile, electrical, maintenance-shop Mobile. 

Dry chemical, cut off electrical power, water Dry chemical. 



OPINION DATA 



Ignition source 

Burning substance 

Successful extinguishing agent. 



Welding, electrical Welding, engine heat. 

Combustible liquids, wiring insulation Combustible liquids, rubber (hose or belt). 

Dry chemical, water Dry chemical, water. 



and for all three major time periods. It is also the primary cause 
of underground injury fires. In recent years, however, engine heat 
has become the leading cause of injury fires. Also significant is 
the increasing number of electrical fires that occur on diesel- 
powered mobile vehicles. Engine heat is the leading ignition source 
for surface fires that were reported, while for nonreportable sur- 
face fires the ignition source was welding. 



BURNING SUBSTANCE 

The most frequent burning substance in reported underground 
fires is timber, followed by combustible liquids and insulation. In 
nonreportable fires, combustible liquids, wiring insulation, and 
timber are involved with about equal frequency. For reported sur- 
face fires, the most frequent burning substance is combustible 
liquids, followed by construction material. In nonreportable sur- 
face fires, combustible liquids are most frequently involved. 

LOCATION 

Reported underground fires occur along haulageways or in 
drifts where electrical or diesel equipment is concentrated. Non- 
reportable fires also occur in these locations more frequently than 
at any other. 

Reported surface fires occur primarily in mill buildings, and 
nonreportable surface fires occur primarily on mobile equipment 



along haulage roads or in the pit area. In recent times, however, 
the reported surface fires are about evenly split between surface 
buildings and surface areas other than buildings. 



EQUIPMENT INVOLVED 

The most frequent equipment involved in reported and non- 
reportable underground and surface fires is mobile type such as 
load-haul-dump vehicles. In underground fires (reported and non- 
reportable), the second most frequently involved equipment is of 
electrical type. For the reported surface fires, it is conveyors in 
the most recent time period (1978-84), and electrical in the 1950-77 
period. 



SUCCESSFUL EXTINGUISHING AGENT 

The most frequently successful extinguishing agent for reported 
fires is water. For nonreportable fires, dry chemical hand-portable 
fire extinguishers are used most often. This is consistent with the 
duration of reportable fires. First attack on a fire is generally with 
a hand-portable extinguisher. If the attempt is successful, then the 
fire is most likely extinguished at the nonreportable stage. If the 
fire has grown in size or initial extinguishing attempts prove un- 
successful, then the fire will probably become reportable and water 
is used as an extinguishing agent. 



CONCLUSIONS 



Several important conclusions can be drawn from the data of 
the metal-nonmetal mine fire study. First and foremost, fire inci- 
dence rates in both surface and underground mines are not declin- 
ing. Despite the considerable efforts of mine safety personnel and 
focused regulatory action, little progress toward reducing the in- 
cidence of fire is apparent. While uncertain reporting during the 
earlier time periods could be blamed for this apparent lack of prog- 
ress, the latest time period (1978-84), for which fire reports are 
believed to be quite complete, is not so easily rationalized, and is 
therefore quite disturbing. One possible explanation, which is sup- 
ported to some extent by the data on ignition sources, burning 
substance, and equipment involved, is that fire hazards are chang- 
ing as mining methods, materials, and equipment evolve. As specific 



fire hazards are recognized and corrected, new mining technology 
introduces other hazards into the workplace. This explanation sug- 
gests that the present level of fire safety effort may not succeed 
in reducing fire incidence rates, and that an accelerated pace of ac- 
tivity with particular focus on newly emerging mining technolo- 
gies is required if incidence rates are to be reduced. 

Another observation also relates to the changing patterns of 
data evident over the three time periods analyzed (1950-67, 1968-77, 
1978-84). Conclusions regarding the relative importance of a given 
fire hazard for one period do not necessarily hold for subsequent 
periods, suggesting the value of regular updates to the fire incident 
data base. Timely collection, analysis, and publication of such data 
will help ensure that fire safety efforts address the greatest needs. 



COMPUTER MODELS OF UNDERGROUND MINE VENTILATION AND FIRES 



By Rudolf E. Greuer 1 



ABSTRACT 

The design of mine fire emergency plans requires that the contamination of mines by fumes, 
and the mutual interaction of fires and ventilation systems be precalculated. Several computer models 
were developed by the Bureau of Mines for this purpose during the last decade. The models advanced 
in sophistication from the transient-state simulation of fume concentrations for the early stages of 
a fire to the steady-state simulation of airflow rates, concentrations, and temperatures for fully 
developed fires, and finally to the complete transient-state simulation of fires and ventilation systems. 
The principal features of these models are described. 



INTRODUCTION 



To calculate the airflow distribution in mine ventilation systems 
as a result of fans, thermal forces, and flow resistances is a for- 
midable mathematical problem. It comprises the solution of twice 
as many equations as there are airways and half of these equations 
are square equations. This sort of problem led to the design of special 
analog computers in the fifties and sixties and, from the early six- 
ties on, to the increasing use of electronic digital computers. With 
the rapidly increasing availability and capacity of electronic digital 
computers, airflow rate and pressure loss distribution calculations, 
commonly called ventilation network calculations, have become 
routine, and a great number of computer programs exist for this 
purpose. Practically all the programs are capable of performing the 
required calculations, although differences exist in how the square 
equations are linearized, the mass conservation law is introduced 
and observed, the fan characteristics are simulated, and the ther- 
mal drafts are considered. All of the programs are based on steady- 
state conditions. A few have rather rudimentary sections for con- 
centration and temperature calculations, as far as the literature allows 
such a judgment. 

Mine ventilation control and mine fire detection and fighting 
are inseparable. Mine fires produce gases and heat, which the ven- 
tilation systems transport through the mines. The gases can be 
poisonous or explosive. The heat can cause ventilation disturbances, 
which take the gases along unexpected routes or affect the forma- 
tion of explosive methane mixtures. 



Of greatest concern in the past were the fire-generated ven- 
tilation disturbances. Ventilation engineers developed a large number 
of methods, by manual calculation, to detect potentially unstable 
airways with airflow reversals in case of a fire. When the analog 
and electronic digital computers became available for ventilation 
planning, they were almost immediately used for this purpose also. 
The expected fire-generated ventilating pressures were manually 
inserted into the network simulations, with their values usually 
obtained from experience or from rough calculations. The mutual 
influence of fire intensities and ventilation conditions were not taken 
into account. If gas concentrations were calculated at all, then only 
for the case that no recirculation existed. All calculations were, as 
in conventional network calculations, based on steady-state condi- 
tions or based on the assumption that no changes with time occur. 
If changes with time happen, one would have transient-state 
conditions. The full potential of computers was for a long time not 
utilized, mainly because the task seemed to be too demanding. 

This attitude changed gradually. During the last 15 yr the 
Bureau supported the development of a number of pertinent 
computer models for the interaction of mine fires and ventilation 
systems. Of these, this paper will describe the models considered 
as particular benchmarks. 



TRANSIENT-STATE CONCENTRATION SIMULATION 



JUSTIFICATION FOR PERFORMING THIS WORK 

The assumption of steady-state conditions may be good enough 
for the determination of airflow rate and temperature distributions, 
because airflow rate changes are caused by temperature changes, 



'Mining engineer. Twin Cities Research Center, Bureau of Mines, Minneapolis, 
MN; professor of mining engineering, MI Technological Univ., Houghton, MI. 



and temperature changes are observed in the immediate vicinity 
of the fire only. Changed airflow rate and temperature distribu- 
tions are, therefore, reached almost instantaneously and time is a 
minor factor. 

Time is definitely an important factor with concentration 
distributions. Gases travel with the ventilation currents, which 
normally means speeds of less than 3 to 4 km/h. Many airways 
downwind of a fire can, for a considerable time after the start of 



the fire, remain clear of gases. In many cases, the increase of airway 
gas concentration happens only gradually before steady-state con- 
ditions are reached. Assuming steady-state concentration distribu- 
tions in fire emergency planning frequently means needlessly 
excluding airways as escapeways, although they are perfectly safe 
for a long time. 



RESULTING PROGRAM VERSIONS 



be helpful for the design of fire escape plans if an early fire warning 
system allows for the evacuation of the mine before the fire has 
progressed beyond its initial stage. 

In the second case, the determination of the airflow rates has 
to precede the concentration calculations. This requires detailed 
information on the ventilation system and some ventilation exper- 
tise by the program user. The determination of airflow rates will 
be described in a subsequent section of this paper. 



A pertinent program for concentration distributions was com- 
pleted in 1981 (7-2). 2 Program sections for the exposure simula- 
tion of escaping miners were completed in 1983 (3). Provisions 
for the cooperation with computer programs for the simulation of 
escape movements of miners, based on warning times and travel 
speeds, were added in 1983 also (4), and mobile contaminant sources 
were included in 1984 (3). 

The program is based on the assumption of constant airflow 
rates. These can be the airflow rates prevailing in the early stages 
of a fire or the airflow rates resulting from the equilibrium of fire- 
generated (thermal) and other (fan, airway resistances) ventilating 
forces. In the first case, very little input data and expertise on the 
part of the program user are required. The only ventilation infor- 
mation needed is data on the network configuration (airway and 
node identification numbers), airflow rates, and airway lengths and 
cross-sectional areas. The program should be of use for all such 
cases where fire-generated ventilation disturbances do not yet occur, 
in particular for the layout of fire detection systems. It should also 



2 Italic numbers in parentheses refer to items in the list of references at the end of 
this paper. 



PROGRAM DESCRIPTIONS 

The simulation of concentration distributions does not involve 
physical principles other than the law of mass conservation. Addi- 
tionally, the assumptions are made that perfect mixing of air cur- 
rents in nodes and no longitudinal diffusion in airways exists and 
that flow velocities in planes perpendicular to flow directions are 
equal. 

The mine atmosphere is divided into control volumes of 
homogeneous concentrations, which advance with the flow through 
the ventilation system. When control volumes meet at airway junc- 
tions, they are extinguished and new control volumes are formed. 
Because many junctions can be reached via different paths with dif- 
ferent amounts of dilution having occurred, the number of control 
volumes can become quite large, depending on the type of ventila- 
tion system. When recirculation occurs, the number can virtually 
become infinity with smaller and smaller concentration differences 
between the newly formed control volumes. Figure 1 gives the 
example of a simple, idealized system with recirculation, which 
indicates the complexity of the problem. 



Contaminant 
source 




After 15 min 



Contaminant 
source 




After 30 min 



Total 



1st 



rong 



Key 
Medium 1®®@©1 Weak 



Absent 



Figure 1 .—Concentration distribution with recirculation contamination. 



The problem of simulation is not one of mathematics but of 
sorting. All control volumes that arrive at a junction have to be 
detected, and the sequence of their arrival has to be determined. 
The new control volumes, which are generated in the junctions, 
have to be advanced into the outgoing airways together with checks 
to see if they affect any other junctions. If they do, part of the com- 
putations have to be repeated. 

The main difficulty in writing this program was, therefore, not 
one of mathematics but one of keeping the computing time in 
tolerable limits. Experience showed that the choice of the time in- 
crement in which the control volumes are advanced is of great im- 
portance. If the increment is too large, control volumes may have 
to be passed through more than one junction, which lengthens and 
complicates calculations. If the increment is too short, a large 
number of advancements has to be performed. The solution was 
to let the program select the optimal length of the time interval as 
a function of the network type. 

The basic organization of the program is shown in figure 2. 
Its core is a triple-nested DO-loop with a joint starting point for 
all three loops. The innermost loop updates state and location of 
the control volumes, an intermediate loop initiates the updating in 
the chosen time intervals, and the outermost loop provides for out- 
put in user-specified time intervals. 

By far the largest part of the program is occupied by the 
innermost loop. It is here that the control volumes are advanced, 
extended, shortened, deleted, or generated (fig. 3). The deletion 
and generation of control volume happens mainly by mixing in net- 



work junctions. It is, for this purpose, necessary to find the proper 
sequence of control volumes arrivals injunctions, which leads to 
a continual process of cross-referencing among junctions. 

Besides initiating the updating of control volumes, the inter- 
mediate time loop has to fulfill a large number of other functions. 
It checks, for instance, if airways have, at least temporarily, reached 
steady -state conditions. It keeps track of available storage for con- 
trol volumes to prevent loss of data, and it organizes the output 
in such a way that repetitious production of results is avoided as 
far as possible. 

Program sections added in 1983 (J) allow one to calculate the 
fume or other contaminant exposure of miners. This looks, in prin- 
ciple, like a simple task that only requires determining the miners' 
locations and the pertinent contaminant concentration. The reality 
is that one deals with moving miners in a moving atmosphere. Dur- 
ing the time intervals that determine the updating of the ventilation 
system, miners may stay in junctions with or without concentra- 
tion changes occurring. They may travel within one or several air- 
ways. In each airway they may travel in the same or the opposite 
direction of the air and with the same speed or faster or slower 
than the air. Their original travel or escape plans will need modifica- 
tion depending on the state of the ventilation system. Miners will, 
in smoke obscured airways and under apparatus, travel with dif- 
ferent speeds than unimpeded miners. This all adds up to quite 
entwined calculations, and the added program sections for exposure 
calculations are almost as lengthy as the original program. 



Airway loop 



Input 



Determine 

optional time 

increment 



Update 

location of 

control volumes 



Airway 
loop 



Time 
loop 



Output 
loop 



Increase time 



Output 

concentration 

distribution 



No 




< 


< 








Update airways 

(delete, generate, 

advance control 

volumes) 






1 






Has airway end 
been reached? 




Go to next 

No 


nirway 






Yes 






' 






Find all control 

volumes affecting 

same node 








1 


' 




Delete, generate, 

advance control volumes 

in proper sequence 






I 








Has a 

airwa 

been re 


nother 
y end 
cached? 


—Yes — 


Go to next 


airway 



Figure 2.— Basic principle of transient-state concentration 
simulation. 



Figure 3.— Update of control volumes in airway loop. 



STEADY-STATE FIRE SIMULATIONS 



It was stated in the ' 'Introduction' ' section of this paper that 
possible ventilation disturbances caused by fires have always been 
of the greatest concern in fire emergency planning. This may be 
less the case where shallow deposits are mined and fans provide 
brisk air currents with significant pressure differences between net- 
work parts. Where mine workings extend, however, over larger 
elevation differences and where the ventilation is weak, such dis- 
turbances have been the cause of large mine disasters. The 1928 
mine fire at Roche-la-Moliere (France), which killed 48 miners, 
is a classical and often quoted example. The first computer model 
(5) on the interaction of mine fires and mine ventilation systems 
focused, therefore, on the assessment of these disturbances. 

To be able to make use of the existing programs for ventila- 
tion network calculations and to obtain a workable model in com- 
paratively short time, steady-state conditions for the ventilation as 
well as for the fire were assumed. This may come close to reality, 
it may in other cases mean a gross simplification. The resulting 
program turned out to be useful nevertheless and became popular 
because of its simplicity. For this reason and since it is part of more 
advanced transient-state computer models, it shall be described here 
in some detail. 



UNDERLYING PHYSICAL PRINCIPLES OF 
STEADY-STATE FIRE SIMULATIONS 

If the terminology of ventilation engineers is used, one can say 
that fires have two effects on ventilation systems: a throttling effect 
caused by the volume increase of the air passing through the fire 
zone; and a natural draft effect caused by the conversion of heat 
into mechanical energy. 

The throttling effect is easy to assess. The energy requirement 
per unit mass to overcome friction losses is proportional to the square 
of the flow velocity. The latter is in turn proportional to the specific 
volume or the absolute temperature. The energy requirements to 
transport air through an airway are, therefore, proportional to the 
square of the absolute temperature. If a fire lets the temperature 
increase, the same amount of energy will transport less air. 

Natural draft effects, or the conversion of heat into mechanical 
energy, can occur where the air changes its pressure and its specific 
volume and where it goes through a cyclic process. Every loop of 
the ventilation system can be considered as a cyclic process and 
can, therefore, develop natural draft effects. The amount of heat 
converted into mechanical energy is, according to the first law of 
thermodynamics, equal to the integral of the products of specific 
volume times pressure change. 

The determination of numerical values of this integral is 
cumbersome because of the continual pressure changes in ventila- 
tion systems and owing to the fact that the specific volume cannot 
be directly measured. A good approximation is possible in most 
cases by establishing a functional relationship between this integral 
and an analogous integral of the products of temperature times eleva- 
tion change, which is easier to determine. 

Precalculations of the throttling and natural draft effect require, 
therefore, the precalculation of temperatures. These are functions 
of the airflow distribution, which determines the oxygen supply of 
the fire, the heat transfer to the airway walls, and the mixing of 
air currents injunctions. The airflow distribution, on the other hand, 
is a function of the temperature-induced throttling and natural draft 
effects. It was this interaction between fires and ventilation systems, 
together with the occurrence of recirculation, which was for many 
years considered to be too complicated for computer simulations. 



RESULTING PROGRAM 

This program combines conventional network (airflow rate, 
pressure loss) calculations with concentration and temperature 
calculations and takes the throttling and natural draft effects 
automatically into account. It tries to establish the equilibrium at 
which fire-generated thermal forces (throttling and natural draft) 
are balanced by the other ventilation forces (fans, resistances). 

Figure 4 shows the program organization. The beginning is 
a network calculation for the prefire state, to ascertain that the input 
data are correct. Based on the type of fuel, fuel loading, and air 
supply, the heat and contaminant production is calculated next. This 
is followed by a calculation of the temperature and concentration 
distribution, based on the original airflow rates. The fire-generated 
thermal forces (throttling and natural draft effect) are then deter- 
mined and inserted into a new network calculation. This sequence 
of calculations is repeated until the calculated thermal forces remain 
constant. This means that the equilibrium has been found and, 
temporarily, steady-state conditions for the airflow rate, pressure 
loss, and temperature distribution have been established. 

This approach can be justified because changes of the 
temperature distribution are observed in the vicinity of the fire and 
short distances downwind from it only. Fire-caused thermal forces 
are, therefore, generated almost instantaneously, and changed 
airflow rate and temperature distributions also establish themselves 



Input 



Network calculation 
(airflow rate and pressure loss) 



Heat and contaminant 
production calculation 



Temperature and 
concentration calculation 



Determination of thermal forces 



Are thermal forces different 
from those used in 
network calculation? 



— Yes J 



No 



Output 



Figure 4.— Basic principle of steady-state simulation of mine 
fire-ventilation interaction. 



10 



almost instantaneously. This does not apply to the concentration 
distributions, however, and only in rare cases to fire intensities. 
Even if the fire intensity does not change, the temperature 
distribution will change with time, since the airway walls will be 
heated and provide less cooling. This is taken into account by con- 
sidering the transient-state nature of the heat transfer, although with 
the simplifying assumption that the equilibrium airflow did prevail 
from the very beginning of the fire. 

PROGRAM DESCRIPTION 

A more detailed plan of the program in the form of a flowchart 
is given in figure 5. It shows that the program has been divided 
into two main parts, network and concentration and temperature. 
To make the program as flexible as possible, network, temperature, 
and concentration calculations can be performed separately or com- 
bined. Methane concentrations are, however, always determined 
when a change in the airflow distribution takes place, since this 
is indispensable for coal mines. 

The network part basically contains a well-proven program for 
conventional network calculations. The term network calculation 



means airflow rate and pressure loss of calculations. The program 
uses the Hardy Cross method, since studies, which were recently 
updated (5), showed that for networks of the size of mine ventila- 
tion systems, this method still gives the shortest calculating times 
when accompanied by a convergency promoting method of mesh 
selection. This can be done by letting airways with large products 
of airway resistances and airflow rates appear in as few meshes 
as possible. The pertinent mesh assembly process occupies a large 
portion of the program. 

So-called fixed quantity airways or regulators, which main- 
tain a constant airflow rate and which are a valuable planning aid, 
have to be converted into regular airways if more than a conven- 
tional network calculation is demanded. In case of an emergency, 
there will be no time to pay attention to the adjustment of regulators 
that keep the airflow rate constant. 

Concentration and temperature calculations are performed 
jointly in the second program part because they have many com- 
mon features. One can think of heat as being a contaminant also, 
and temperature changes caused by heat influx or by mixing are 
calculated in the same way as concentration changes. 

The flowscheme program section uses airway and node iden- 
tification numbers to establish which air currents go into the same 



Network 



Start 



Network calculation demanded? 



No 



Yes 



Output of 

network Input 



Read and try to 
complete input 



Incomplete 



Assemble meshes 



Satisfy junction equation 



Caluclate natural ventilation pressure from 
temperature and evaluation of junctions 



Apply cross correction 
simulate fan characteristics 



Is this the first 
network calculation? 



t 



Yes 



Convert regulators 



Print results of first 
network calculations 



No 



No 



Concentration and Temperature 



Temperature or concentration 
calculation demanded? 



No 



Yes 



Read and try to 
complete input 



Incomplete 



Calculate CH4 production 



Print Input 



_L 



Establish flowscheme 



■e 



_L 



Perform roadway calculations 



Estimate properties 
of recirculated air 



Perform junction calculations 



No 



4 



Do estimates for 
recirculated air satisfy? 



Yes 



Calculate fire— generated 
thermal forces 



Equilibrium reached? 



Yes 



_L 



Print results 



Stop 



Figure 5.— Flowchart of steady-state simulation. 



11 



junction to be mixed and which air currents leave from the same 
junction, and, therefore, have the same properties at their 
beginnings. 

Concentration and temperature changes in roadways can be 
caused by entering methane, by fire-produced contaminants and 
heat, and by heat entering or leaving the airway. Injunctions they 
can occur because of mixing of air currents with different concen- 
trations and temperatures. Both processes are fundamentally dif- 
ferent, and two separate program sections have been provided. 

The calculations start at a node with known temperatures and 
concentrations. Normally this will be the surface or some place in 
the intake airways. With the conditions at their beginnings thus 
known, roadway calculations are conducted for all airways leaving 
this node. Next, a check for junctions where the conditions of all 
entering airways are known is performed. If found, the entering 
air currents are mixed and roadway calculations are performed for 
all airways leaving this junction. 

This process is interrupted when recirculated air enters a junc- 
tion. In this case the alternating roadway and junction calculations 
cannot be continued, since concentrations and temperatures of the 
recirculated, entering air are not known. The difficulty is overcome 
by using an iteration method. Starting out with estimated values 
for the properties of the recirculated air, the roadway and junction 
calculations are continued as if they were known. With this assump- 
tion, the values, which are then obtained for the recirculated air 
by the successive roadway and junction calculations, are next used 
as new, better estimates. The process is repeated until estimated 
and calculated values agree. Airways with recirculation are placed 
in a special list in the output to draw attention to the fact that they 
carry potentially contaminated air into intake airways. It is surprising 
how much recirculation exists in many mines without being 
recognized as such. 

Heat and combustion products developed by fires can either 
be estimated by the user and extend into the program, or can be 
determined by the program as functions of the oxygen supply of 
the fire. The heat exchange with the airway walls is calculated with 
the help of Fourier's equation of thermal conduction, for which 
solutions have been built into the program. 

A crucial role in these calculations is played by the rock 
temperature, which is a function of virgin rock temperature and 



the airway history. It is possible, in principle, to determine the 
temperature distribution in the rock and to provide an accurate 
solution of Fourier's equation, if the airway history is known. 
Normally this will not be the case. 

Because fire emergency plans deal with short time spans, com- 
pared with the age of the airway at least, and since only thin layers 
of rock surrounding the airways are affected by the temperature 
changes, it seems to be accurate enough to work with effective rock 
temperatures for this layer. These are close to the normal air 
temperatures, modified by the temperature difference caused by 
convection, and are determined by the program. If better informa- 
tion exists, it can, of course, be fed into the computer and used. 

The fire-generated thermal forces are the throttling and natural 
draft effects. They are determined by making use of the calculated 
temperature distributions. 



PROGRAM USE 



The program was written for routine applications by practical 
ventilation engineers but without violating physical laws or simplify- 
ing facts of practical importance. The amount of necessary input 
data has been kept small and checks are performed for their com- 
pleteness and for such errors as occur most frequently. Where they 
do not influence the results significantly, user-stated average values 
can be used to reduce the input. 

The output provides a listing of airflow rates and pressure losses 
in airways, and of methane and contaminant concentrations and 
temperatures at airway ends. Concentrations and temperatures are 
also listed for junctions. Recirculation paths and airways with airflow 
reversals, as well as roadways and junctions with critical condi- 
tions, are additionally listed to alert the user to potential danger 
zones. 

Because of its capabilities to calculate concentrations and 
temperatures and to take thermal forces into account, the program 
is not only used for fire emergencies but also for ordinary ventila- 
tion planning purposes. There exist several versions of it, developed 
by program users. The program has found substantial use for fire 
simulations in high-rise buildings during the last few years. 



COMPLETE TRANSIENT-STATE SIMULATIONS 
OF VENTILATION SYSTEMS 



The transient concentration distribution programs are useful 
in many ways. The assumption of time-constant airflow rates is 
justified for the early stages of a fire when a weak fire does not 
influence the airflow distribution. When, at a later stage, more in- 
tense fires may interact noticeably with the ventilation system, with 
the fire intensity controlled by the air supply to the fire, and with 
the airflow distribution affected by the fire intensity, the interac- 
tion should be taken into account. Correspondingly, a program was 
written for this condition in which, for the sake of simplicity, steady- 
state conditions were assumed. 

The shortcoming of this program is that it determines the state 
of a ventilation system at the end of a specified time interval only, 
with the assumption that the airflow distribution at the end of this 
interval prevailed from the beginning of the fire throughout the time 
interval. This may be true, but it may be a simplification of reality. 

There are several different approaches for a complete transient- 
state simulation of ventilation systems. The natural one to use seems 
to be a finite difference method based on Newton's second law. 
One starts with the momentum balance of small elements of air 
masses to set up the governing equations, and ends up with a set 



of simultaneous equations that have to be solved for the whole 
system in each time increment. This method was discarded after 
a lengthy trial for the following two reasons: 

1 . With the involvement of simultaneous equations, the calcula- 
tion load increases rapidly when a ventilation network becomes 
large. 

2. For most cases in mine ventilation, the airflow can assume 
steady state in a time period short enough to justify the application 
of steady-state theories. 

The chosen approach was then to consider the transient proc- 
ess as a sequence of short-time steady-state processes during each 
of which an equilibrium between ventilating pressures (fan and 
thermal pressures) and airflow-rate-produced pressure losses exists. 
If the temperature distribution changes, the equilibrium will be 
disturbed and a new airflow distribution will result. The new 
equilibrium for the end of the time interval is found by a sequence 
of alternating temperature, thermal pressure, and network calcula- 
tions. The results of the last calculation serve as the input for the 
following calculation. 



12 



Neither the airflow rates at the beginning of the time interval 
nor at its end are representative for the whole time interval. With 
the assumption that the airflow rate changes during the time inter- 
val follow a linear pattern, the arithmetic average of the two values 
is used to represent the average airflow rate. 



PROGRAM ORGANIZATION 

An interval-oriented simulation technique, which updates its 
data base in every prefixed time interval, was adopted (6). An event- 
oriented simulation would be more efficient, but with a transient- 
state simulation of ventilation systems conditions change constantly, 
which makes events undistinguishable. The control volume 
approach, with control volumes being blocks of air of uniform com- 
position, was retained. In other words, the time is divided into a 
series of time intervals, the airflow into air segments. 

The control volume approach causes difficulties with 
temperatures and water vapor concentrations, which are not 
uniform. Their distributions are calculated, and discontinuities be- 
tween the front and rear ends of adjoining control volumes are 
averaged out. 

If data records are set up for every airway in a system, a ma- 
jority of the efforts would be unnecessary because the variation of 
ventilation parameters in most airways that were not directly af- 
fected by the hot fumes are at most of a secondary significance in 
shaping the airflow distribution in the system. It is assumed that 
the mean air temperature and airflow resistance of an airway will 
remain unchanged if no significant changes in ventilation condi- 
tions occur. The criteria to judge a significant change include 
whether fumes ever got into the airway and whether drastic condi- 
tion changes (air temperature change larger than 1° F, or fume con- 
centration change larger than 0.1 pet) happened in the beginning 
junction of the airway. When no significant change happens in an 
airway, its original resistance and mean air temperature are taken 
as the values in the present time interval. 

Figure 6 shows a very simplified flowchart of the transient- 
state program. The real program is quite lengthy, with approxi- 
mately 4,500 lines of FORTRAN 77 statements and with 42 
subroutines. Since it is in many respects a combination of the 
transient-state concentration program and the steady-state network 
and temperature calculation model, it encounters the same dif- 
ficulties that these two models have to face. Two important im- 
provements were made: a model for the fire zone or the fire 
behavior; the consideration of mass transfer in the form of water 
and water vapor. 



COMPUTER MODEL OF FIRE ZONE 

Great difficulties are still encountered in simulating the fire 
behavior. The steady-state program uses highly simplified, em- 
pirically obtained functional relations between contaminant and heat 
production of a fire and the air passing through it. This is essen- 
tially just a way to define the potential strength of a fire rather than 
a simulation of fire behavior. 

Although many data and observations on mine fires have been 
collected, there does not yet exist a satisfactory mathematical model 
for them. Different researchers focused their attention on different 
parameters. Some came up with empirical or semiempirical models, 
which may be useful for a limited range, but all of which suffer 
from lack of generality. 

Combustion processes are not easy to describe. They are self- 
sustaining exothermic reactions, which provide heat energy and 
combustion products at a rate depending on fuel, prevailing 
temperatures, pressures, and reactant concentrations. Physical proc- 





Start 




1 


Input 
















Mesh formation 




1 








Temperature distribution 
before the mine fire 






Parameter 
updating 




























Intermediate 
data transfer 








Airflow reversal? 










i 














Fire Source 
parometer updating 




Change fan 
operation? 








1 








ZE 


in each airway 




Change 
resistance? 


1 






1 






Updating parometer record 
for airway ending 
















1 








Update junction 
conditions 








1 








Advance new control 
segments into airways 








1 








Calculate natural drafts 
and throttling effects 








1 






Balance network 




I 






Detect fume— free 
evacuation routes 










1 








Output 




e out 




Tin- 




End 





Figure 6. — Flowchart of transient-state ventilation simulation 
program. 



esses exert a significant, sometimes dominant, influence on the fire 
also through the transport of matter and energy. 

Reasonable models must take reaction kinetic, mass, and heat 
transfer in their various forms into account. Such models are pres- 
ently developed (7) and it is hoped that one day they can be helpful 
in the design of realistic emergency plans. Because of the large 
amount of calculation work required by the digital simulation, they 
should not yet be used for routine emergency planning, when 
simplified methods can do almost as well. 

Using the example of timber, which seems to be the by far 
most researched fuel for mine fires, the organization of the com- 
puter models shall be described. The fire zone is divided along the 
airway into segments of a few inches width. The segments are 
radially, or perpendicular to the airway axis, divided into sections. 
The boundaries between segments and sections are marked by nodes. 
Updating of conditions in segments, sections, and nodes is done 
in time increments of 1 s or less. Since a fire zone can be tens or 
even hundreds of feet long, and every update demands the solution 
of several equations per segment, section, and node, the calcula- 
tion effort is quite large. 

In a typical segment as shown in figure 7, section 1 represents 
the control volume in the bulk flow of air current. Section 2 stands 
for the control volume in the wall layer where the products of wood 
pyrolysis burn. Its thickness is taken as a twentieth of the airway 
hydraulic radius, roughly comprising the eddying sublayer and the 
buffer zone, passing by a small percentage of the total mass flow 
of air. 



13 



Sections 



Nodes 



3 ft 



h 



T 



H 



r Rock 



y Wood 
layer 



y Wall 
layer 



y Bulk 
flow 



Figure 7.— Location of sections and nodes in fire zone 
segment. 



Sections 3 and 4 are the wood layer close to the surface, sec- 
tion 5 is the rest of the wood. The total thickness, X, of the wood 
support is calculated from the wood loading per foot of airway 
length. Recommended values for the thickness of sections 3 and 
4 are 0.1 *X for X < 76.2 mm (0.25 ft) or 7.62 mm (0.025 ft) for 
X > 76.2 mm (0.25 ft). 

Sections 6 and 7 are in the surrounding rock. A total thickness 
of 914 mm (3 ft) is recommended for both sections. This is well 
above the thickness of the rock layer in which the temperature is 
affected by a fire. 

The pyrolysis of wood is expected to occur in the region be- 
tween nodes 3 anad 5 according to Arrhenius' equation. As the 
pyrolysis is mainly and sensitively controlled by the local 
temperature, denser subgrids of 20 strips are put into this region 
with the local temperatures in strips being determined through ex- 
ponential interpolation between the adjacent node temperatures. 

It is necessary to divide the airflow into segments also, although 
these segments do not need to be as short as the wall segments. 

The basic logic of the program can be outlined as follows. Sets 
of equations are developed to describe local energy balances. They 
contain endothermic heat of pyrolysis, exothermic heat of combus- 
tion of volatiles and char, conductive and radiative heat transfer, 
enthalpy flux accompanying mass movement, and external ignition 
heat input. The pyrolysis is regarded as a function of local 
temperature using Arrhenius' equation. If thermal decomposition 



has taken place, it is a function of its pyrolysis history also. The 
exchange of volatiles, oxygen, and combustion products between 
bulk flow and wall layer is calculated using mass transfer theories. 
The oxygen supply is compared with the volatile emission. If surplus 
oxygen exists, it is used for the combustion of char residue. 

Many of the equations for node temperatures are nonlinear. 
They are linearized through Taylor's expansion with temperature 
corrections as new unknowns. These are determined using the Gauss 
elimination method and node temperatures are obtained through an 
iterative approach. 



CONSIDERATION OF WATER VAPOR TRANSFER 

There exist by now a large number of approaches to predict 
the combined dry heat transfer by conduction within the rock and 
by convection from the rock to the air. None of these approaches, 
however, take the parallel mass transfer of water and its temperature 
effect satisfactorily into account. 

This is regrettable because it is recognized that even seemingly 
dry airways are to a considerable extent influenced by water migra- 
tion, evaporation, or condensation. Small water quantities can, due 
to the large latent heat of water, have great effects. 

The fact that no convincing mathematical descriptions of the 
simultaneous heat-mass transfer exist indicates the complexity of 
the problem. 

Past attempts to describe the effects of water on mine air 
temperatures fall roughly into two categories. One of them uses 
statistical tools to interpret systematical field-measured data for em- 
pirical relationships among temperature, humidity, and other ven- 
tilation parameters. It does not attempt to explain the nature of their 
dependence or to draw generally valid conclusions. Rather it offers 
useful equations of localized significance. 

The other is a semiempirical technique that tries to derive func- 
tional relationships between temperature and other ventilation 
parameters. A number of loosely defined factors, coefficients, and 
more or less justified relations are introduced for this purpose to 
provide for general applicability. There exist, however, so many 
assumptions, which are neither theoretically sound nor universally 
tenable, that the usefulness of this approach has to be limited in 
range or a lack in accuracy has to be expected. 

The computer model reported here tries to follow a rigorous 
analytical approach and a rigorous analytical solution can be obtained 
(8). Since the calculation of numerical solutions turns out to be a 
very time-consuming task, approximations for short time periods, 
with which one is concerned in emergency planning, were developed 
also. 

Additional difficulties arise from boundary conditions. Water 
can evaporate or condense on airway walls with pertinent changes 
in wall temperatures. It can condense as fog in the bulk flow of 
the air with a pertinent change in the temperature gradient between 
airway walls and air. To be properly considered, these wall 
temperature changes have to be recorded. To keep the amount of 
record keeping small, so-called stations in intervals of a few hun- 
dred feet are established, for which the correct data are recorded. 
For the space between stations, interpolations are used. 

It is very much desirable to find a more simplified approach 
for considering the influence of water transfer on mine temperatures. 
Work in this direction continues. But to simplify sensibly, the 
unsimplified facts have to be known first. 



14 



SUMMARY 



A transient-state computer model for ventilation systems has 
been the ultimate goal toward which all program development 
strove. The transient-state concentration model and the steady-state 
fire and ventilation interaction model were essentially compromises 
between what was desirable and what was feasible at the time. 

The lack of a transient-state ventilation model, which allows 
the quantitative prediction of ventilation patterns in thermally 
disturbed mine ventilation systems, has been overcome. Morever, 
the program described in this paper has been made user friendly 
and a personal computer version has been developed, which should 
make it helpful to every engineer dealing with fire or ventilation 
problems. 



The simulation of the transient processes in fire zones and the 
simulation of water transport is still cumbersome, because it was 
attempted to take all significant factors, like chemical kinetics, heat 
and mass transfer, unabridged and unsimplified into account. 
Extensive testing of these newly developed program sections with 
experimental data may show that justified simplifications are 
possible. 

Since only the well-established laws and principles of thermo- 
dynamics, fluid mechanics, heat transfer, and mass transfer have 
been used, the transient-state computer model should be theoretically 
sound. It has given plausible results so far. 



REFERENCES 



1. Bastow, K.R. Real-Time Simulation of Contaminant Flow Through 
Mine Ventilation Networks Under the Influence of Mine Fires. M.S. Thesis, 
MI Technol. Univ., Houghton, MI, 1979, 306 pp. 

2. Greuer, R.E. Real-Time Precalculation of the Distribution of Com- 
bustion Products and Other Contaminants in the Ventilation System of Mines 
(contract JO285002, MI Technol. Univ.). BuMines OFR 22-82, 1981, 263 
pp.; NTIS PB 82-183104. 

3 . . A Study of Precalculation of Effect of Fires on Ventilation 

System of Mines (contract JO285002, MI Technol. Univ.). BuMines OFR 
19-84, 1983, 293 pp.; NTIS PB 84-159979. 

4. Sheng. J. Determination of the Cumulative Exhaust Effects of Diesel 
Powered Equipment Underground. M.S. Thesis, MI Technol. Univ., 
Houghton, MI, 1984, 159 pp. 



5. Greuer, R.E. Study of Mine Fires and Mine Ventilation; Part I. Com- 
puter Simulation of Ventilation Systems Under the Influence of Mine Fires 
(contract SO241032, MI Technol. Univ.). BuMines OFR 115(l)-78, 1977, 
165 pp.; NTIS PB 288 231. 

6. Chang, X. Digital Simulation of Transient Mine Ventilation. Ph.D. 
Thesis, MI Technol. Univ., Houghton, MI, 1987, 162 pp. 

7. Chang, X., and R.E. Greuer. A Mathematical Model for Mine Fires. 
Ch. in Proceedings of the 3d U.S. Mine Ventilation Symposium. Soc. Min. 
Eng., AIME, Littleton, CO, 1987, pp. 453^62. 

8. . Simplified Method To Calculate the Heat Transfer Between 

Mine Air and Mine Rock. Paper in Proceedings of the 2d U.S. Mine Ven- 
tilation Symposium, ed. by P. Moussett- Jones. A. A. Balkema, 1985, pp. 
429-438. 



15 



MINE FIRE DETECTION SYSTEMS: A PRIMER 



By Charles D. Litton 1 



ABSTRACT 

This Bureau of Mines paper discusses a simple approach to the design and implemen- 
tation of mine fire detection systems. Topics include state-of-the-art technology in fire sensors 
and systems, a discussion of the hazards that different types of fires can produce, and some 
simple guidelines for determining the level of protection required for different mining areas 
and types of combustibles used or stored. 



INTRODUCTION 



The successful performance of a fire detection system 
depends upon its ability to detect the presence of a fire rapidly 
and reliably. It should be sufficiently rapid so that there 
remains enough time to either safely evacuate a mine or 
successfully extinguish and control the fire, or both. It should 
be reliable so that false alarms are minimized without sacri- 
ficing sensitivity of time response. Systems should also be 
durable enough to withstand the mine environment over long 
periods of time while maintaining a high degree of reliability. 
Maintenance and calibration of the system should be simple 
and not require the expenditure of excessive time and labor to 
keep it operational. 

Many factors influence the design of a fire detection 
system, such as the potential sources and modes of ignition, 
the types of combustibles involved, and the quantities of 
combustibles available for fire growth and flame spread. Mine 
air ventilation influences the growth and spread of a fire and 
also serves to transport the products of combustion to other 
areas of a mine, remote from the fire. Fire sensor alarm 
thresholds place limits on the sizes of fires that can be 
detected. The time to respond to a detected fire, either in terms 



of evacuation or control and extinguishment, place additional 
time constraints on the design of fire detection systems. 

These factors interact with each other, often in complex 
ways, and it is the definition and understanding of these 
factors and their interactions that hold the key to the design of 
adequate and reliable fire detection systems. The level of 
understanding of the problem has increased dramatically 
within recent years and it is the intent of this paper to discuss 
the progress that has been made and how the information can 
best be used to implement fire detection systems in under- 
ground mines. 

To address the potential for fire detection systems, a 
framework must be created with which the design of these 
systems can evolve. This framework should address not only 
the detection of fires, but also the levels of detection that may 
be required for different areas of mines. The different levels of 
detection are based upon the types of fires that may be 
expected, and their potential hazards. To begin to understand 
the problem, it is necessary to have knowledge about the fires 
that are most likely to occur and the sensors and systems 
available for their detection. 



MINE FIRE SENSORS AND SYSTEMS 



Most mine operators and mine safety personnel are 
familiar with minewide monitoring systems that are available 
from a number of different manufacturers. Mine fire detection 
is a major component of these systems, which now use CO fire 
sensors. Currently, the Bureau-developed submicrometer par- 
ticle smoke detector is being developed commercially for use in 
such systems and should be available in the near future. Both 
CO and smoke detectors are product-of-combustion fire sen- 
sors. These sensors are designed primarily to detect the CO 
and smoke liberated from either a smoldering or flaming fire 
into the mine ventilation airflow where their concentrations 



1 Supervisory physical scientist, Pittsburgh Research Center, Bureau of Mines, 
Pittsburgh, PA 



are diluted and carried along with the airflow. The diluted 
concentrations of CO and smoke that are present within the 
ventilation airflow depend upon the rates at which they are 
produced relative to the quantity of air that mixes with these 
combustion products. 

For fixed sensor alarm thresholds, it becomes apparent 
that very small fires may be detectable at low ventilation 
airflows, while at higher airflows larger fires are needed to 
reach the same alarm level. Sensor sensitivity, then, is an 
important parameter, but setting of alarm thresholds most 
often depends upon the mine background level of either CO or 
submicrometer particles. Alarm thresholds should be signifi- 
cantly higher than these background levels to protect against 
false alarms of the sensors. High false alarm rates can destroy 



16 



confidence in a fire detection system, rendering the system 
useless. At the other extreme are sensors that are too insensi- 
tive, with high alarm thresholds incapable of detecting fires in 
their earlier stages of development. Figure 1 provides some 
insight into this type of phenomena. At low values of the 
alarm threshold level to background level ratio, false alarms 
significantly reduce the confidence in the system. As this ratio 
increases, so does confidence in the system, until, at some 
point, the sensor becomes much less sensitive to small fires and 
confidence in the system again decreases. 

For CO fire sensors, confidence greater than 90 pet is 
usually achieved when this ratio is in the range of about 3 to 
15, assuming that the background CO levels typically are 1 to 
5 ppm and an alarm threshold level of 15 ppm is used. 
Assuming a minimum background level of 1 ppm, increasing 
the alarm threshold to greater than 15 ppm decreases the 
sensitivity of the system and at much larger values, the system 
becomes incapable of achieving its desired function. A similar 
analysis can be made for smoke particles. 

The electrical and electronic components necessary for 
the system to operate must also exhibit a high degree of 
reliability. Their function is to provide power to the sensor, 
accept sensor signals, supervise the sensors to assure proper 
operation, and record and display information and alarms. 
This is usually done in two stages. Outstations, to which a 
number of fire sensors and other transducers (such as veloci- 
meters) are electrically connected, are installed underground. 
Sensor power is supplied by these outstations and sensor 
signals accepted. This information is then transmitted to a 
central processing unit, usually located aboveground at a 
permanently staffed location. The central station accepts the 
data from undergound, records it, and if desired, displays it 
on a video screen. Many levels of sophistication are possible 
for the total system. What level is used depends upon the 
particular mine and its needs, as well as the cost of the system. 

At the outstation level, most commercial systems are de- 
signed to accept signals from a variety of transducers. This is 
important from a fire detection standpoint when sensors, other 
than CO or smoke, such as optical sensors are used. Optical 
sensors, such as ultraviolet or infrared sensors, are fast respond- 
ing detectors (approximately milliseconds or faster) that sense the 
radiant energy emitted by a fire. They are recommended for use 
in applications where instantaneous response is critical, such as 
fuel storage areas and power centers. Such sensors may not be 
available from the manufacturer of the monitoring system, but 
can be purchased separately and are easily interfaced with most 
monitoring systems. It is usually wise to determine in advance if 
a monitoring system can use a variety of transducers, or if 
interfacing them to the system is a simple procedure which is 
relatively inexpensive. 

Mine monitoring systems and, in particular, those systems 
used primarily for fire detection have shown tremendous 
growth in use within the past 2 to 3 yr. At the same time, users 
of such systems are beginning to express some dissatisfaction 
with the calibration and maintenance requirements of the 
sensors. This is particularly true for systems installed in large 
mines where the number of sensors required can be quite high. 
Past research by the Bureau has shown that an alternative 
approach to that of fixed-point fire sensors is a pneumatic 
monitoring system. 

For this type of system, individual tubes extend from a 
central pumping and detection station to the desired monitor- 
ing locations, as indicated in figure 2. This central station may 
be located aboveground or at some convenient location under- 
ground. When located underground, this central station is 
analogous to the outstation of the fixed-point mine monitoring 




A/B 

Figure 1 .—Variation in the percent confidence in a mine fire 
detection system as a function of the ratio of alarm threshold 
level to background level (A/B) for product-of-combustion fire 
sensors. 



Tubes from 
monitoring locations 



Sample 
pump 




.Solenoid valves 
or equivalent 



(sensor Ij - 
^Sensor 2 J- 



( Sensor nj- 



► Exhaust 



Scavenger 
pump 



Exhaust 

Figure 2.— Schematic of central detection and pumping sta- 
tion for use with a pneumatic fire detection system. 



17 



system. At the central station (fig. 2), all tubes except for the 
one being sampled are continuously purged by a scavenger 
pump. Each tube has its flow diverted through a three-way 
solenoid valve at fixed time intervals to a smaller sample pump, 
which presents the tube contents to one or more sensors for 
measurement of CO, smoke, or other gases of importance. 

This type of system is attractive for a number of reasons. 
First, the cost of the system is less than that of a fixed-point 
monitoring system. Second, because all components, includ- 
ing sensors, are located at the same place, calibration and 
maintenance requirements are significantly less. Third, mea- 



surement of any additional gas at all locations is easily 
accomplished by adding a single gas analyzer to the system. 
This system is ideally suited for continuous monitoring of 
gobs, mined-out areas, return airways, etc. Through proper 
choice of tube diameters and pump capacities, the system can 
be designed for rapid response. 

The use of this system suffers from the fact that it is not 
commercially available as a system. However, all components 
necessary are readily available, off-the-shelf items and design 
of these systems is easily accomplished. 



FIRE CHARACTERISTICS 



Fires produce heat, light, and combustion products (gases 
and smoke). These represent measurable quantities that, when 
detected at low levels, signal the presence of a developing fire. 
Fires develop through three distinct stages. The first stage is 
the preflaming, smoldering stage where fuel is pyrolyzed and 
produces products of incomplete combustion and fuel vapors 
that are eventually ignited, resulting in open, flaming combus- 
tion. Smoldering fires produce little heat or light, but copious 
amounts of smoke, CO, and often other gases. Further, the 
smoldering stage of combustion may last for prolonged peri- 
ods of time, hours or days, or it may be of much shorter 
duration depending upon the source of ignition. 

The transition to open, flaming combustion signals the 
beginning of the second fire stage. During this stage, detect- 



able levels of heat, light, and products of combustion are 
produced as the fire grows in intensity. The rate of growth may 
also vary dramatically. Liquid pool fires reach a stage of 
steady-state burning in a matter of several seconds, while fires 
of coal rubble or other solid combustibles grow in size at a 
much reduced rate. In general, this stage of the fire remains 
localized, consuming the combustibles present in the general 
vicinity until, at some point, the fire intensity is sufficient to 
produce propagation or rapid flame spread down an entry. 

The propagating fire represents the final stage of a 
developing fire and represents the most severe hazard. Once 
this stage is reached, entire mines can be devastated. Little, if 
any, opportunity exists for either escape or control of the fire. 



FIRE HAZARDS 



Smoldering fires, or fires that develop to the point of 
smoldering due to spontaneous combustion or other modes of 
self-heating, produce little heat or light, but are more appro- 
priately characterized by the liberation of smoke, CO, and 
other potentially toxic gases. External sources, such as over- 
heated equipment or electrical shorts, may produce smoldering 
periods that last for minutes or hours, while smoldering fires 
of spontaneous origin may have much longer periods of 
prolonged smoldering before flaming combustion is reached. 

The primary hazard associated with smoldering combus- 
tion is the toxicity of the smoke and gases that are produced. 
Knowledge of this toxicity is lacking and represents an area of 
intensive research. However, both CO and smoke are major 
products of smoldering fires and either CO or smoke detectors 
are capable of providing for early warning of this stage of fire 
development. Research into the relative toxicities of smolder- 
ing fires will provide the information necessary to evaluate the 
response of CO and smoke sensors relative to the potential 
toxic hazard. It is sufficient to say, for the moment, that CO 
and smoke sensors provide for early warning of most smolder- 
ing, incipient fires, but the relationships between their alarm 
level thresholds and the potential toxic hazard are not well 
understood. 

The hazards that result from flaming fires are functions of 
the ratio of the fire size to the ventilation rate, expressed as 
Q f /v f A, where Q f is the heat release rate of the fire (kW), v f is 
the ventilation velocity (m/s), and A is the average entry 
cross-sectional area (m 2 ). Three major hazards result from 
fires: 

1. Toxicity associated with the gases and smoke products 
of combustion. 



2. Reduced visibility (smoke obscuration) from the smoke 
particles. 

3. Fire propagation or rapid flame spread along the 
combustible surfaces of an entry. 

The relative toxicities of products produced from a variety 
of mine combustibles is an area of research currently being 
addressed. It is known that CO is a major toxic gas component 
of the combustion products. Others have been found, such as 
HC1 from materials containing chlorine. For purposes of this 
paper, it will be assumed that CO represents the primary toxic 
hazard reaching an incapacitating stage at a concentration of 
approximately 1,500 ppm (0.15 pet) or greater. The fire size 
necessary to produce this concentration is given by 



(Q r /v f A) T > 300.0, 



(1) 



where the subscript T denotes the toxic hazard. 

The production of copious amounts of smoke severely 
reduces mine visibility and can significantly reduce the poten- 
tial for escape. It is generally recognized that when the smoke 
level reaches a concentration sufficient to reduce visibility by 
approximately 84 pet over a 12-ft path, an extreme hazard 
exists. To produce this reduced visibility, a smoke particle 
concentration of approximately 225 mg/m 3 is required. The 
fire size necessary to produce this concentration is given by 



(Q f /v f A) s > 150.0, 



(2) 



where the subscript S denotes the smoke hazard. 

Fully developed, sustained fire propagation occurs when 
the hot gas temperatures from a fire are sufficient to ignite 



18 



exposed surfaces of combustible materials along an entry. 
Both theory and data indicate that this condition is reached 
when the fire size is defined by 

(Q r /v f A) P > 450.0, (3) 

where the subscript P denotes the propagation hazard. 

The times that are required for fires to reach these critical, 
hazardous levels can vary dramatically with the type of fuel 
involved, the amount of fuel available, and its distribution. 
Fires that involve spills of flammable liquids or liquid pools 
develop within seconds to these levels. For instance, a spill of 
liquid heptane, if ignited, will reach the critical smoke hazard 
level in 5 to 10 s, and the critical propagation hazard in 
approximately 40 s. Fire detection systems for these cases 
should also contain means for automatic suppression and 
extinguishment. 

At the other extreme are fires that are preceded by 
prolonged periods of smoldering. Such fires generally occur in 
areas of the mine that are free from sources of external heat 
and energy, and are due to spontaneous, self-heating. The 
detection of these fires need not be so rapid. Several minutes or 
hours will generally suffice, but if left undetected, the conse- 
quences can be just as dramatic as those of the liquid pool fire. 

Between these two extremes lie fires, usually of solid 
combustibles, which are ignited by external sources of heat and 
energy. Periods of smoldering may be of short duration or 
nonexistent. Once flaming occurs, the fire intensity begins to 
increase, although generally at a much slower rate than for a 
liquid pool fire. Clearly, once flaming occurs, there exists only 
a finite amount of time available to detect the fire, evacuate 
personnel, and extinguish the fire before the situation becomes 
critical, lives are lost, and property devastated. 

Figure 3 illustrates the times that are involved during the 
growth stages of a fire involving solid combustibles (solid 
line). At the origin of the fire, alarm levels of smoke or CO are 
produced within a minute or so, perhaps even before flaming if 
there is enough smoldering combustion. Once these alarm 
levels are produced, the ventilation airflow carries them away 
from the fire to a fire sensor that must respond and issue an 
alarm. All of this must occur in approximately 15 min, or less. 
This leaves about 50 min before the fire reaches a stage where 
smoke obscuration becomes critical. In about another 60 to 70 
min, the CO becomes life threatening, and within another 60 
min the fire is spreading rapidly throughout the mine. 

For comparison, the dashed line shows the growth rate of 
a liquid pool fire with a surface area of 1.5 m 2 . In less than a 
minute, the fire is large enough to begin propagating. 



10.000 



■a 1,000 
ui 

CO 

< 



100 



10 
0.01 



~-<^ 
<&** 



_Propagation y 

CO 

critical 
Smoke « 
critical 



Alarm 

+ 



Detectable products 
(smoke, CO) 




0.10 



1.0 10.0 

TIME, min 



100.0 



1,000 



Figure 3.— Graphical representation of the sequence of events 
common to development of many underground mine fires. 



From this discussion, it is apparent that fires of differing 
origins require different levels (or times) of detection. For 
smoldering fires, detection times of a few to several hours will 
suffice. For fires of solid combustibles driven by external 
sources of heat and energy, detection times of a few minutes 
are required. And for fires of flammable liquids or fires in 
areas where significant combustible materials (solid, liquid, or 
gas) are stored or used, detection must be within a few seconds 
and often must contain a means for automatic suppression of 
the fire. This analysis points to a need for defining those areas 
of a mine applicable to the three levels of detection. 

It is proposed that a mine be divided into three distinct 
classes when considering the design of minewide fire detection 
systems. The first class (class I) areas are those areas where the 
risk of fire is high and also where the consequences are most 
dramatic. Typical class I areas would include fuel storage 
areas, fuel transfer areas, maintenance areas, and other areas 
where significant quantities of combustibles or flammable 
materials are stored or used. 

The second class (class II) areas are those areas where the 
risk of fire and its consequences are not so great, but where 
there exist sufficient external ignition sources. Examples of 
class II areas would include conveyor belt entries, track entries, 
or other entries within which potential external sources of 
ignition exist and on which routine mining operations depend. 

The third class (class III) areas are those areas where the 
risk of fire is the least. In general, these areas do not contain 
sources of external ignition, but rather, fires develop via 
spontaneous heatings or self-heatings independent of external 
sources. Examples would include return airways, mined-out 
areas that are feebly ventilated, and other areas that in general 
are remote and not crucial to the routine operation of the 
mine. 

CLASS I AREAS 

In general, these areas are relatively small, localized areas 
where the risk of fire and its consequences are greatest. Rapid 
detection, often coupled with automatic suppression capabil- 
ity, within seconds is generally warranted. To achieve this level 
of detection, optical sensors, or a well-defined array of 
thermal sensors, should be used as the primary detection 
system. 

While these areas may be well-defined, localized areas, 
they are also under the influence of some degree of ventilating 
airflow. Consequently, the possibility exists that these fires 
may contaminate the ventilating airstream before detection 
and suppression can be achieved. To provide some degree of 
protection against this possibility, it is also recommended that 
one or more product-of-combustion sensors be located down- 
stream of these areas in the primary ventilating airflow to 
detect any leakage of combustion products. Further, fires 
within these areas may, in some instances, be preceded by a 
prolonged smoldering stage for which no heat or light is 
produced. To provide protection for this possibility, a second- 
ary product-of-combustion sensing system should be used in 
conjunction with the primary optical or thermal sensing 
system. 

If the ventilation flow within a class I area is well defined, 
then the product-of-combustion sensor should be located 
within the area and immediately upstream of the point at 
which the ventilation flow from this area and the primary 
ventilating air mix. If the ventilation pattern within the area is 
poorly defined, then a system of two or more sensors spaced 
according to National Fire Protection Association standards 
(or equivalent standards) should be used. 



19 



To summarize, class I areas represent extremely high risk 
areas for which the potential consequences of fire are the most 
severe. Rapid detection of the initial flaming stages dictates the 
use of an optical or thermal sensing system. In many areas, the 
system should also contain automatic fire suppression capa- 
bilities. An additional secondary product-of-combustion fire 
sensing system should be used in parallel with the primary 
system to protect against the possibility of smoldering fires. 
One or more additional product-of-combustion sensors should 
be installed within the primary ventilating air immediately 
(approximately 50 to 100 ft) downstream of a class I area to 
protect against potential contamination of the primary air. 



CLASS III AREAS 

At the other extreme lie the class III areas for which the 
risk of fire is the least. However, if these areas are left 
unprotected, then the consequences of fire can be just as 
dramatic. In general, these areas are characterized by the 
absence of external ignition sources. As a result, fires that 
develop in these areas can be expected to develop over longer 
periods of time, most probably because of spontaneous com- 
bustion or self-heating resulting from other causes. 

Since no heat or light is produced during smoldering, the 
fire sensor of choice is a product-of-combustion sensor. The 
choice of which product-of-combustion sensor to use (CO or 
smoke sensor) depends upon the most probable fire scenario. 
If a mine has a history of spontaneous combustion or if the 
area to be protected consists of caved-in areas or gobs, then a 
CO sensor is the logical choice. This arises from the fact that 
CO is the best indicator of spontaneous combustion fires. 
Smoke is usually generated at a later point in time relative to 
CO. If smoldering fires other than those of spontaneous origin 
can be expected within a class III area, then smoke sensors are 
the obvious choice to provide an earlier warning. For smolder- 
ing fires of unknown origin, a combination of CO and smoke 
sensors may be warranted. 

But where are the sensors to be located? Excluding for the 
moment return entries as class III areas, the bulk of class III 
areas can be expected to be remote areas, normally weakly 
ventilated and for which ventilation patterns may be ill de- 
fined. However, it is generally known that ventilating air goes 
into an area and comes out. To determine if combustion 
products are produced within this area, it is prudent to make a 
differential measurement. This means that a sensor should be 
located within the airsteam entering a class III area and one 
out by the area to measure what comes out. The difference is 
what is produced within that area. 

For primary return airways, sensors should be located 
immediately upstream of each junction of airways where flows 
mix. If, for instance, three splits of return air mix or merge at 
some point, then a sensor should be located within each split 
entry just prior to that point. An alternative to this approach 
would be to locate a single sensor immediately downstream of 
the mixing point. However, the concentration of CO or smoke 
measured at that point will be reduced because of dilution by 
clean air from the unaffected splits. This implies that a more 
sensitive detector may be warranted if only one is used. The 
single-sensor approach also does not provide the information 
of which split entry contains the fire, and significant time may 
be lost in locating the fire. 



Optimum location of mine fire sensors for the protection 
of class III areas is an area of research that needs to be 
addressed more explicitly. For these areas, the primary concern 
is the detection of fires in their spontaneous or smoldering 
stages for which function either CO or smoke sensors are the 
primary sensors to be considered. These products are carried 
from point to point by the ventilating airflow. As a result, the 
optimum placement of sensors depends critically upon accu- 
rate definition of the airflow patterns within these areas. 

A tool that is ideally suited for this application is the 
Michigan Technological University mine (fire) ventilation 
code. Other ventilation codes exist, but this code is designed to 
provide information that is more relevant during a mine fire 
emergency. First, the code is capable of predicting ventilation 
patterns from a minimum of information. Second, the code is 
capable of predicting the rates of contamination of all areas of 
the mine by fire combustion products. The first capability can 
provide the information necessary to optimize fire sensor 
locations. The second capability can provide information 
relevant to the most prudent escape route to be followed during 
a fire emergency. 

The code is available for use by the mining industry. Its 
potential for use as a tool in the design of fire detection 
systems is a subject of current research. 



CLASS II AREAS 

Firmly entrenched between class I and class III areas of a 
mine are the class II areas. For these areas, there exist many 
potential sources of ignition — overheated equipment, electri- 
cal arcs and shorts — to name two of the most common. These 
areas are characterized by the presence of equipment necessary 
to the normal mining activity — conveyor belt haulage systems, 
electrical trolley entries, track haulage entries, etc. They are 
also entries, some of which may be long or short, that are 
frequently used and which are critical to the mine operation. 
Class II areas may contain class I areas, but these are 
separately protected. 

The times that are available before critical hazards are 
reached in class II areas were previously discussed. Unlike class 

I or class III areas, the time to respond to a fire within a class 

II area is a significant factor, which must be addressed during 
the design stages of the system. 

Ventilation is usually well defined in class II areas, and it 
is the ventilation which transports the combustion products 
from a fire to a fire sensor. A good estimate of the spacing 
requirements between sensors can be made by multiplying the 
desired maximum response time by the velocity of the air 
within an entry. For instance, if a response time of 15 min or 
less is desired in an entry where the velocity is 100 ft/min, then 
the sensor spacing should be no more than 1,500 ft. 

The detection of fires based on product-of-combustion 
sensors is one approach where the number of required sensors 
is relatively low. The sensors also provide a capability for the 
detection of any smoldering combustion, which would precede 
the onset of flaming. Thermal or optical sensors may also be 
used, but their numbers increase dramatically in order to 
provide the same level of detection. Such sensors would also be 
insensitive to any smoldering combustion that might occur. 



20 



DISCUSSION 



There exist many areas of mine fire detection research that 
need to be addressed. The toxicity associated with products of 
combustion is not well understood. Advances in detector 
technology need to be made, especially for applications in 
diesel-operated mines where the diesel combustion products 
often overwhelm the fire products and render detection sys- 
tems to a low level of reliability. The effects of ventilation 
patterns on developing fires and the converse effects of a fire 
on the ventilation system need further study in order to 
quantify these effects. Mine ventilation codes offer significant 
potential for use as tools in defining optimum sensor locations 
and in the preplanning of evacuation and escapeways. 

Much remains to be done before the understanding of 
mine fire problems and detection systems is complete. Yet, the 



level of information and resources available at the current time 
indicate that there exists the potential for significant improve- 
ment in mine fire detection systems. As the level of under- 
standing of these problems increases, so will the levels of 
detection that can be provided increase. 

The design and implementation of fire detection systems 
need not be overwhelming. It can begin by identifying those 
areas of mines that are most in need of protection and building 
from there. The framework presented here serves as a guide as 
to what is practical and possible with the present level of 
technology. 



21 



FIRE DETECTION SYSTEMS FOR NONCOAL UNDERGROUND MINES 



By W. H. Pomroy 1 



ABSTRACT 

Early fire detection can be a critical element in a mine's overall strategy for dealing with fire 
emergencies. This paper describes results of Bureau of Mines research to evaluate, through long- 
term in-mine tests, the performance of fire detection devices in all four fire detection categories: 
heat, flame, smoke, and fire gas. The basis for detector selection, as well as installation and 
maintenance practices to help insure reliable detector performance, are also provided. 



INTRODUCTION 



Early detection of an underground mine fire can significantly 
increase the likelihood of survival for underground workers and 
minimize the time required for a mine to return to production after 
such an emergency. Early fire detection (or ideally, detection dur- 
ing the incipient stage), may enable evacuation before smoke and 
toxic fire gases grow to life -threatening concentrations or block 
visibility. Also, since smaller fires are more easily extinguished 
and less hazardous to fight, early fire detection contributes to faster 
and more effective fire control. Improved fire control minimizes 
fire damage, thereby permitting production activities to resume more 
quickly. 

The National Fire Protection Association classifies fire detec- 
tion devices into four general categories, as follows (7): 2 

Heat detector Detects abnormally high tempera- 
tures or rate of temperature rise. 

Flame detector Detects the infrared (IR), ultra- 
violet (UV), or visible radiation 
produced by a fire. 



Smoke detector 



Fire gas detector 



Detects the visible or invisible par- 
ticles of combustion. 
Detects gases produced by a fire. 



All four fire detector types have application in underground 
metal and nonmetal mines. The purpose of this paper is to describe 
recent Bureau research to field test each of the fire detector types 
in typical underground metal and nonmetal mine applications. The 
intent is not to provide an exhaustive listing of all detector varia- 
tions and potential uses, but rather to illustrate with case examples 
typical uses of each type of detector. Site-specific conditions such 
as mining method, airflows, combustible materials present, provi- 
sion for fire warning and evacuation, and mining equipment used, 
would determine the suitability of a particular detector for a given 
application. 



HEAT DETECTION 



Industrial-grade thermal fire detection devices are generally 
characterized by high reliability and durability but low maintenance 
requirements, even when used under the harshest conditions. 
Clearly, these attributes are desirable for mining applications. 

One limiting feature of thermal detectors is that they rely on 
convected thermal energy for response. The distance between the 
detector and the fire, the relative spatial orientation and placement 
of the detector relative to the fire, and local air currents profoundly 
affect detector performance. Heat detectors are, as a result, the 
preferred choice for small, well-defined fire hazards, especially if 
they are enclosed, such as electrical boxes. In order to provide for 



■Group supervisor. Twin Cities Research Center. Bureau of Mines, Minneapolis, 
MN. 

2 Italic numbers in parentheses refer to items in the list of references at the end of 
this paper. 



large area coverage, numerous closely spaced detectors are required. 
A common example of large-area thermal detection in the mining 
industry is the typical conveyor belt fire detection system mandated 
for underground coal mines. Spot-type, thermal detectors, spaced 
at 125-ft intervals along the entry provide early warning of a belt 
fire. 

A string of spot-type detectors arrayed in a similar manner in 
a mine shaft is a feasible approach to shaft fire detection, however, 
the use of a line-type device would offer superior performance. A 
line-type device senses the heat from a fire at any point along its 
length. It can be thought of as spot-type detection in the limiting 
case where the distance between adjoining detectors equals zero. 

A prototype line-type thermistor strip fire detection system for 
mine shafts was developed by the Bureau and installed along the 
entire length of the 1 ,200-ft main production shaft of a salt mine 



22 



in Detroit. MI (2). The thermistor strip detection system selected 
was the Alison Control A888-M106 fire detection system. 3 

The control unit was housed in a National Electrical Manufac- 
turers Association (NEMA) 12 enclosure. A separate annunciator 
was provided in a NEMA 9 enclosure. The system provided two 
independent, adjustable levels of alarm (prealarm and alarm) that 
were annunciated at both the control unit and annunciator. The lo- 
cation of the overheated area was also indicated in feet above or 
below ground level on a digital display at the annunciator. 

The sensor was completely supervised. An abnormal condi- 
tion was indicated at the control unit and annunciator if an open 
circuit or short occurred anywhere along the entire length of the 
sensor. All interconnections between the control unit and annun- 
ciator were also supervised. 

Should the system lose ac input power, the power supply was 
automatically disconnected and standby batteries (located at the 
bottom of the control unit) were switched in automatically. The 
system contained a battery charger that automatically maintained 
the batteries fully charged when ac power was present. 

The sensor cable consisted of stainless steel tubing containing 
a specially formulated ceramic thermistor core. A center wire 
imbedded in the core ran the entire length of the sensor. 

The sensor center-wire-to-case resistance exhibited a negative 
temperature coefficient. This meant that as the temperature in- 
creased, the resistance of the sensor decreased exponentially. 



'Reference to specific brand names does not imply endorsement by the Bureau of 
Mines. 



It was this decrease in resistance that was sensed by the alarm 
instrumentation. 

The 40-ft sensor sections were connected in series to form 
two sensor circuits, each 600 ft (15 sections) in length. The entire 
sensor length and all three junction boxes were coated with a 
heavy polymer jacket for further protection from the corrosive 
atmosphere. Figure 1 illustrates the layout of the control panel, an- 
nunciator, and detection cable. 

The system was completely installed by a three-person crew 
over a 4-day period. Because this shaft is the main mine exhaust, 
the air is laden with salt. This highly corrosive atmosphere is 
detrimental to the operation of electrical systems, necessitating 
great care in hermetically sealing each detector segment intercon- 
nection with a silicone adhesive-sealant. All external parts of the 
detector wire and connections were stainless steel, which was fur- 
ther protected with a corrosion-resistant fluorocarbon polymer 
jacket. Following installation, the system was functionally tested. 
At a known elevation in the shaft, a propane torch was used to 
heat a section of the detection cable. The prealarm and alarm func- 
tions operated properly and the hotspot indicator displayed the cor- 
rect elevation. 

The system was operated continuously for 14 months without 
hardware failure. Once during that period, a lightning strike at the 
headframe structure caused a momentary alarm, however, the 
system returned to normal operating mode without further incident. 
These test results are significant because they indicate that the hard- 
ware and installation precautions are suitable for this worst case 
corrosive environment. 



Headframe and crusher building 




System control panel 



Thermister strip 
lower detection zone 



Mining level 



Figure 1.— Schematic showing mine shaft linear thermal fire detection system control panel, annunciator, and detection cable. 



23 



FLAME DETECTION 



Flame detectors, either UV or IR, are preferred where ex- 
tremely fast response to a fire is essential because of the likelihood 
of very rapid fire growth or explosion, as with class B liquid fuels. 
Typically, flame detectors are connected directly to fast-acting 
automatic fire or explosion suppression systems in such high-hazard 
areas as fueling platforms, petrochemical plants, and hyperbaric 
chambers. An example of the use of flame detectors in underground 
mines is the Bureau-developed fueling area fire protection system, 
which utilizes UV detection to trigger the release of twin agent (dry 
chemical and aqueous film-forming foam) fire suppressants (3). UV 
detection was selected over IR to minimize false alarms. Sources 
of false alarms include arc welding for UV detectors and hot sur- 
faces or gases for IR detectors. It was determined that the system 
could be disabled during welding operations, but that vehicular 
traffic, a probable source of hot surfaces and gases, could not be 
avoided in the fueling area. 

The Detronics U7602 UV detector was selected for the sys- 
tem (fig. 2). This detector responds to the wavelengths of light in 
the UV range of 1,850 to 2,450 A. The electronics are housed in 
an explosion-proof enclosure constructed of two screw-together 
coaxial cylinders that are heavily plated for corrosion resistance. 
The UV viewing area is a 90° cone. Two digital alarm modes are 
provided: one closed relay for fire alarm and one open relay for 
dirty lens alert. The detector operates on 24-V dc power. The detec- 
tor utilizes a Geiger-Mueller type tube to sense UV radiation (2, 4). 

The detector is also equipped with an UV test lamp that mon- 
itors the integrity of the optical lens and deenergizes a relay when 



the surfaces become obstructed with oil, dirt, or dust. The UV test 
lamp emits UV radiation that passes through the lens, reflects off 
a beveled reflecting ring mirror, passes back through the lens and 
into the tube. 

No operational problems were encountered with the detector 
during extensive laboratory tests, or during in-mine fire tests and 
long-term in-mine endurance tests at two mines. In each in-mine 
installation, two detectors were cross-zoned within the control unit 
to minimize false alarms. Cross-zoning requires that both detec- 
tors respond to a fire at the same time to actuate the suppression 
subsystem. The control units were provided with internal adjusta- 
ble time delay circuits to enable attending personnel to abort the 
discharge of suppressant if necessary (for example, a false alarm 
due to welding in the fueling area). 

Fire testing involved igniting diesel fuel contained in a 2- by 
3-ft pan placed under a load-haul-dump vehicle mockup. Because 
one of the goals of the in-mine fire tests was to evaluate the ex- 
tinguishing effectiveness of the twin agent suppressant system, the 
control unit was operated in the abort mode during these tests. This 
was necessary because operation in the normal mode would have 
resulted in almost instantaneous detection and suppressant release. 
In the abort mode, the fires achieved full fuel involvement and thus 
represented a more severe test for the suppression system. How- 
ever, the detectors did respond almost immediately to the test fire 
flames, and no deterioration in detector performance was observed 
during the long-term endurance test period. 



SMOKE DETECTION 



One of the earliest products of incipient combustion is sub- 
micrometer sized particulates, or smoke. Smoke detection systems 
that are capable of reliably detecting these particulates are extremely 
valuable because fires can be detected before they reach the flam- 
ing combustion stage. With the aid of such systems, emergency 




Figure 2.— UV flame detector used in fueling area fire protec- 
tion system. 



procedures, such as personnel evacuation and fire-fighting efforts, 
can be undertaken at the earliest opportunity, often before the fire 
poses a direct threat. A complete prototype smoke detection sys- 
tem was designed, fabricated, and installed in an underground cop- 
per mine in Arizona for prolonged testing and evaluation. 

The submicrometer particulate detector selected was the Anglo 
American Electronics Laboratory Becon MK IV ionization type 
combustion particle detector (fig. 3). The cylindrical outer casing 
of the Becon detector is made from nylon-dipped stainless steel to 
ensure detector longevity in the highly humid and corrosive under- 
ground mine environment, and to provide a radiation shield. 

Towards the lower end of the Becon detector are vertical rec- 
tangular ports, which allow mine air to enter the ionization chamber. 
The ports in the stainless steel shield are internally overlapped by 
a nylon-dipped stainless steel baffle plate, which shields the areas 
outside the detector from direct radiation, reduces the effects of 
high velocity airflow in the ionization chamber, and causes a mix- 
ing of the mine air inside the ionization chamber. 

Internally, the Becon MK IV particle detector contains a 
shielded single ionization chamber, a radioactive source, an ion 
collecting electrode (grid), and a current amplifier. Because of the 
inherent corrosive nature of the underground mine atmosphere, all 
internal components of the detector are made of plastic or are 
hermetically sealed. 

The radiation source, which ionizes the air within the ioniza- 
tion chamber, is a sealed glass vial containing 5 mCi of krypton 
85 gas. The design and operating principal of the Becon MK IV 
is described in reference 2. 

The location for mounting the Becon MK IV detector should 
be near or on the downwind side of a potential fire hazard area, 
however the ventilation air velocity in the chosen area should not 
exceed 1,200 ft/min. 



24 




Figure 3.— lonization-type combustion particle detector install- 
ed in underground mine. 

Because the Becon MK IV detector has no moving parts, very 
little maintenance is necessary. Periodic examination of the elec- 
trical cable for breaks or frays and calibration are all that is required. 

The system in the mine consisted of 10 detection instruments. 
Each detector was equipped with a digital telemetry module 
(mounted in the detector cap) to convert the detector analog output 
to a digital word for transmission of the detector value along with 
a unique address and verification words to the system control unit. 
A microcomputer system control, a line interface control module 



for communication to the computer through an industry standard 
protocol (RS232C), a disk drive to store the control program and 
detector output records, a color video display with graphics to 
highlight alarms, and a printer to provide hard copy of alarm and 
fault messages were also provided. 

The detectors were linked to the system control by a single- 
pair closed-loop telemetry circuit. Connecting outstations in a closed 
loop minimizes cable costs and installation time and provides a 
redundant signal path for uninterrupted signal transmission in case 
of a broken telemetry line. 

The system was completely installed by a four-person crew 
over a 1-week period. Minor debugging was required following 
installation because of problems with several telemetry modules, 
however, the necessary repairs were effected on site during the week 
following the installation. 

The system operated for approximately 1 yr with a simplified 
control program while the final version of the control software 
was developed and debugged. During this period, system opera- 
tion was limited to a video display of real-time detector outputs 
and an audible alarm and printout whenever any detector output 
exceeded its individually programmed alarm threshold. 

The system control software provided video graphics of the 
detector locations on color mine maps, simple instructions, and 
three-key coded function commands and alarm, fault, and trouble- 
shooting messages. Following the on-screen prompts and using the 
simple three-key commands, operators could display one of the three 
mine maps covering the system, change any sensor alarm threshold, 
display 72-h sensor history in tabular or graphic form, and manually 
control the printer. 

During the first 3 months of system operation, numerous false 
alarms were issued. The detector-mounted digital telemetry mod- 
ules, which are susceptible to low voltage conditions, were found 
to be the cause. Boosting line voltage slightly corrected the prob- 
lem. This detector has been used by the Bureau in several other 
research installations and performance in every instance has been 
excellent (2, 5-5). 

Following this initial "burn-in" period, the system operated 
for approximately 24 months. During this period, three abnormal 
events (smoking rubber drive pulleys on two pumps and a smok- 
ing electrical controls enclosure) were detected by the system. 



FIRE GAS DETECTION 



Like smoke detection, gas detection is particularly useful for 
large-area coverage in underground mines where ventilation air- 
flow can transport airborne products of combustion great distances 
from the source of a fire. 

Bureau mine fire detection research utilizing fire gas detec- 
tors has included two basic fire detection system configurations: 
the pneumatic tube bundle approach and the fully electronic tele- 
metry approach. Pneumatic detection involves sampling the mine 
atmosphere through a network of plastic tubes that terminate at a 
central analytic station equipped for gas monitoring. The electronic 
felemetry approach involves the placement of detection devices at 
each underground location to be monitored. Detector outputs are 
transmitted to a central control point over electronic telemetry lines. 
Such systems may consist of any number of detection devices, with 
one or more detectors installed at each monitoring point. An ex- 
ample of each approach is described in the following sections. 



PNEUMATIC DETECTION APPROACH 

Although pneumatic detection systems have been used in 
aboveground occupancies for many years (factories, ocean vessels, 



etc.), their use in underground mines is fairly recent. Practical, 
mineworthy pneumatic detection systems were developed about 20 
yr ago in the United Kingdom for the detection of slowly develop- 
ing spontaneous combustion fires in coal mines. Considerable 
research effort has been directed toward application of this approach 
in North American coal mines (9-1 J), however, it was never widely 
accepted by industry outside the United Kingdom. The Bureau has 
recently completed a research program to design and in-mine test 
a rapid response pneumatic fire detection system tailored to the 
unique requirements of multilevel metal mines. The prototype 
pneumatic detection system consisted of three primary subsystems: 
air sampling, detection, and control. Each subsystem is described 
in the following sections. 



Air Sampling Subsystem 

The air sampling subsystem was required to draw samples of 
the mine atmosphere from various underground locations through 
plastic tubes to an analytic station where the presence and level of 
combustion gases could be determined. Vacuum pumps were pro- 
vided in the analytic station for this purpose. All electrical and 



25 



mechanical equipment would thus be centralized for ease of main- 
tenance and removed from the harsh underground environment for 
improved performance. Polyethylene tubing was selected for its 
durability, flexibility, light weight, and low cost. A main bundle, 
consisting of the sample tubes surrounded by Vi in of thermal in- 
sulation and a tough outer neoprene jacket, was installed in the shaft, 
with individual tubes branching off on various levels to specific 
monitoring locations. 

Water traps were required to prevent the accumulation of water 
in the sample lines. Such accumulations could seriously impair 
sample flows. This effect would be particularly acute where warm, 
moist mine air is drawn through sample tubes that are routed in 
intake air near a mine opening or other area where the air temper- 
ature is below the dewpoint of the sample. Accumulated water could 
also freeze, further compounding the problem. 

Standard water traps were modified for this purpose using a 
specially designed two-way check valve. During normal operation 
(i.e., under vacuum), a vacuum check valve prevented air leakage 
into the water trap and hence, dilution of the sample. To empty 
the traps, the system was periodically cycled into a pressure mode, 
wherein the entire tubing network was pressurized with compressed 
air. The water traps were equipped with float-type check valves 
that permitted accumulated water to be blown out by compressed 
air pressure but then sealed against pressure loss once the trap was 
empty. 

Two vacuum pumps were required for system operation: a 
purge pump and a sample pump. The purge pump maintained a 
constant flow in the lines, exhausting to the atmosphere. The sam- 
ple pump drew air samples from each line in sequence, exhausting 
to the detection instruments. 

Three-way, solenoid-operated valves having low flow resistance 
were installed in each line. The valves were sequentially cycled 
by the system control to direct sample gas from one line at a time 
to the sample pump and gas analyzers, while the remaining flows 
from the other 1 1 lines passed through the purge pump and were 
exhausted. 

Detection Subsystem 

It was determined that both CO and C0 2 detection should be 
incorporated into the prototype detection system. Although both 
gases are formed in most fires, one or the other gas would likely 
predominate, depending on the type of fire. Thus, analysis of the 
ratio of the two gases, along with other data such as the known 
fire zone, would enable a characterization of any fire that might 
occur. A system incorporating two detectors would also be in- 
herently more reliable than one utilizing a single detector, as the 
two detectors would provide a degree of redundancy. 

Detectors based on the operating principle of nondispersive IR 
(NDIR) absorption were selected for the prototype pneumatic detec- 
tion system. Radiation from an IR source is passed through a cell 
containing the sample of gas to be analyzed, and is absorbed by 
the gas present. A filtered IR detector responds to this change in 
radiation and its output is compared with a reference cell, condi- 
tioned by suitable electronics, and read out on an appropriately 
marked meter. A stable reading is generally obtained in 2 to 5 s, 
followed by a rezero in 3 to 4 s. 

NDIR detectors are quick and accurate, sensitivity to 1 pet of 
full scale can be achieved. For CO, a detection range of to 100 
ppm was utilized, with the resulting sensitivity being 1 ppm (2 pet 
of the 50-ppm threshold limit value (TLV)). For C0 2 , a detection 
range of to 2,500 ppm was utilized, with the resulting sensitivity 
being 25 ppm (0.5 pet of the 5,000-ppm TLV). Until recently, 
NDIR detectors were confined to laboratory use only, as they were 
too delicate to withstand even moderate temperature and humidity 
variations. Present models are more robust and are designed for 



limited field exposure. Because the detection instruments in a 
pneumatic system are centralized in a relatively clean environment, 
this was judged to be an acceptable application for NDIR detection. 

Control Subsystem 

The detection system was controlled by a 64-K RAM micro- 
computer and associated hardware. The computer operated in a 
process control mode to monitor the operational status of the vari- 
ous detection system components (pumps, analyzers, etc.), cycle 
the solenoid valves in the proper time sequence, initiate gas analyzer 
calibration and water trap blowout routines, and issue alarm and 
trouble warnings. The computer also stored system data and pro- 
vided the user with several menu-selectable video display, system 
output, and system control options. 

To insure accuracy in measuring gas concentrations, the system 
also controlled a pair of automatic gas analyzer calibrators. When 
activated by the computer, these calibrators automatically standard- 
ized the gas analyzers using calibration gases contained in high- 
pressure gas storage cylinders. 

Figure 4 shows the system control room containing the pumps, 
valves, gas analyzers, autocalibrators, and computer. 



In-Mine Test Results 

The prototype pneumatic detection system was installed and 
functionally tested at a multilevel underground zinc mine in Og- 
densburg, NJ. The system was installed by a four-person crew over 
a 4-week period. The system monitored 18 locations through 12 
tubes. 

A total of 39,900 ft of tubing was used in the prototype system. 
About 80 pet of the total was contained in the main bundle in the 
shaft, with the remaining 20 pet being single tubes that branched 
off on various levels and led to specific monitoring locations. Tubes 
contained within the main bundle were color coded and numbered 
to facilitate installation and subsequent system layout changes. 

Overall system performance was found to be satisfactory dur- 
ing the 3-yr in-mine evaluation period, in that the capability to 
monitor and record elevated CO and C0 2 levels at the desired 
underground locations, and warn mine personnel when these levels 
exceeded specified thresholds, was demonstrated. 

System operations were closely monitored throughout the 3-yr 
evaluation period. Late afternoon excursions in both the CO and 
C0 2 levels, corresponding to end-of-shift blasting operations, were 
a daily qualitative check on system performance. 

The provision for blowing water from the sample tubes proved 
to be quite effective. Inspection of the tubes shortly after the sys- 
tem was commissioned, but prior to the installation of the water 
traps, revealed accumulations of water at low points where the 
tubes sagged. Following installation of the traps, such accumula- 
tions were not completely eliminated, however enough of the water 
was removed by the traps that system performance was not im- 
paired by accumulated water. Purging the tubes with high-pressure 
(80 psi) compressed air cleared both the traps and other water 
accumulations. 

Leaks in the tubing system were noted throughout the test 
period. These leaks, along with longitudinal mixing of gases within 
the tubes, made the direct determination of gas concentration at 
the sampling point impossible. However, each sampling point could 
be calibrated with a test gas of a known concentration and subse- 
quent measurements adjusted accordingly in the control software 
if a precise quantitative measure of concentration is needed. 
Significantly, the leaks that did occur did not affect the transit time 
of the samples in the tubes, meaning that a timely warning would 
be received even if a system was prone to leaks. All tubing system 



26 




Figure 4.— Control room for pneumatic detection system showing pumps, valves, gas analyzers, autocalibrators, and computer. 



leaks were traced to faulty connections between individual lengths 
of tubing. Prevention of leaks could be accomplished by using longer 
lengths of tubing (therefore requiring fewer connections) and us- 
ing better connectors. 

Overall system maintenance requirements were minimal. Mine 
personnel made a visual check of the system about once per week, 
and calibration gas tanks required replacement every 2 months. At 
that time, the analyzers were manually calibrated and the vacuum 
pumps were checked for dirt and debris in their in-line filters. 

Reliability of the gas analyzers exceeded the project's design 
goals. After 6 months of operation, the analyzer cells were removed 
and cleaned; however, no subsequent maintenance was required 
or performed. Autocalibrator reliability also exceeded design goals, 
with the only problem being a faulty control valve, which was 
discovered and replaced during installation. 

The computer control functioned continuously throughout the 
test period without difficulty. Power outages at the mine site oc- 
curred occasionally, however, the self-booting feature of the con- 
trol software functioned as designed, and the system automatically 
returned to a state of full operational readiness when power was 
restored. 

ELECTRONIC TELEMETRY APPROACH 

The electronic telemetry approach is by far the more common 
type of fire gas detection. Indeed, with respect to current industry 
practice, the pneumatic approach is limited to deep coal mines in 
the United Kingdom, whereas the electronic telemetry approach 



is widely used in the major mining districts, both coal and non- 
coal, worldwide (2, 12-13). The electronic telemetry approach 
enables the monitoring of any gas, particulate, or condition (air 
velocity, direction, temperature, humidity, etc.) for which a detec- 
tion instrument exists. Equally important are the communication 
and control functions, such as telephones, pump switches, fans, 
and doors, that can utilize the same telemetry lines. Where telephone 
lines are already installed, fire detectors can usually be added at 
a nominal cost without affecting voice communication. 

Telemetry -type fire gas detection systems, especially CO 
systems, are becoming increasingly popular in both coal and non- 
coal mines in the United States. An example of this technology is 
a fire gas detection system installed by the Bureau at an Idaho silver 
mine for long-term evaluations. The detectors were installed on the 
1900 level of the mine in the main exhaust. In this location, the 
detectors were subjected to quite severe environmental exposures: 
air velocity of 800 ft/min, air temperature of 85 ° F, and saturated 
humidity. 

The system included both NDIR analyzers (of the type used 
in the pneumatic detection system) and electrochemical detectors 
for the continuous monitoring of CO. The units were placed side 
by side at the same location for comparison purposes. 

Because the NDIR unit is designed for, at most, limited field 
exposures, it was necessary to enclose it in an environmental hous- 
ing. Because of the extreme humidity at the test site, a stainless 
steel NEMA 12 box was selected. The analyzer was placed inside, 
along with a sample pump and calibration gas tanks. The finished 
assembly was 2 by 3 by 4 ft and weighed over 160 lb. 



27 



Three electrochemical cell units were included in the system: 
Ecolyzer 5000, Ecolyzer 4000, and MSA 571 . All three detectors 
utilize the electrochemical properties of a fuel cell to sense CO. 
The electrochemical sensor is constructed of three electrodes— the 
sensing electrode, the reference electrode, and the counter 
electrode — all suspended in an acid solution. The materials to be 
chemically reacted are CO and O gases from the mine ambient air. 
These gases diffuse into the acid (or in the case of the Ecolyzer 
4000. are pumped into the fuel cell by an air pump) solution and 
ionize. 

The CO is electrochemically oxidized at the sensing electrode 
while O reduction occurs at the counter electrode. The ion concen- 
tration in the acid solution, because of the dissolved gases, is pro- 
portional to the concentration of CO in the air; likewise, the cur- 
rent flow through the cell is proportional to the ion concentration 
in the solution. Therefore, the current flow through the cell is pro- 
portional to the CO content of the air. This current flow is then 
amplified and compensated for temperature before it is sent to the 
sensor control. All three units are supplied by the manufacturer 
in rugged environmental housings and are intended for installation 
where harsh environmental exposures are expected. Figure 5 shows 
an Ecolyzer 4000 installed underground. 

All the detectors were linked to the surface through a digital 
telemetry system similar to that used for the smoke detection system 
described earlier. 

Throughout the 1-yr test period, all units functioned properly. 
Calibration was performed approximately 24 h after powerup, as 
previous research had shown that considerable drift could be ex- 
pected during the first 24 h of operation. All units tracked closely, 
with end-of-shift blasting a daily qualitative performance indicator. 
Based strictly on performance, no basis could be established for 
choosing one detector over another. However, cost considerations 
clearly favored the electrochemical units. 




Figure 5.— Electrochemical cell-type CO detector installed in 
underground mine. 



SUMMARY AND CONCLUSIONS 



Early fire detection through the use of specialized fire detec- 
tion instruments can play an important part in a mine's overall mine 
fire emergency plan. It provides sufficient advance warning of an 
emergency to enable safe evacuation and effective fire fighting. 



Numerous commercially available detection devices have been tested 
in a variety of mine settings with such consistent success that early 
fire detection and warning must be considered a proven technology 
whose application can be recommended industrywide. 



REFERENCES 



1. National Fire Protection Association. Standard on Automatic Fire 
Detectors. National Fire Code NFPA 72E-1984, 1984, 48 pp. 

2. Pomroy, W. H., and R. E. Helmbrecht. Design and Operation of 
Four Prototype Fire Detection Systems in Noncoal Underground Mines. 
BuMines IC 9030, 1985, 25 pp. 

3. The Ansul Co. Improved Fire Protection System for Underground 
Fueling Areas. Volumes 1 and 2 (contract H0262023). BuMines OFR 
120-78, 1977, 325 pp., NTIS PB 288298; BuMines OFR 160-82, 1981, 
111 pp., NTIS PB 83-114744. 

4. Baumeister, T., and L. S. Marks (eds.). Standard Handbook for 
Mechanical Engineers. McGraw-Hill, 7th ed., 1967, pp. 16-29. 

5. Pomroy, W. H. Spontaneous Combustion Fire Detection for Deep 
Metal Mines. BuMines IC 9144, 1987, 25 pp. 

6. Johnson, G. A., and D. R. Forshey. Inmine Fire Tests of Mine Shaft 
Fire and Smoke Protection Systems. BuMines IC 8783, 1978, 17 pp. 

7. Stevens, R. B. Demonstration of a Mine Shaft Fire and Smoke Pro- 
tection System for Coal Mines (contract H0100017. ESD Corp.). BuMines 
OFR 116-85, 1985, 294 pp.; NTIS PB 86-146933. 



8. . Mine Shaft Fire and Smoke Protection System. (Final 

Report.) Volume II— Validation Testing and Cost-Effectiveness Evalua- 
tion (contract H0242016, FMC Corp.). BuMines OFR 73(l)-78, 1987, 210 
pp.; NTIS PB 284 166. 

9. Litton, C. D. Design Criteria for Rapid-Response Pneumatic Monitor- 
ing Systems. BuMines IC 8912, 1983, 23 pp. 

10. Hertzberg, M., and C. D. Litton. Pneumatic Fire Detection With 
Tube Bundles, J. Fire and Flammability. v. 9, Apr. 1978, pp. 199-216. 

11. Chakravorty, R. N., and R. L. Woolf. Evaluation of Systems for 
Early Detection of Spontaneous Combustion in Coal Mines. Paper in the 
Proceedings of the 2d International Mine Ventilation Congress (Reno, NV, 
Nov. 4-8, 1979), ed. by P. Mousset-Jones. Soc. Min. Eng. AIME (Little- 
ton, CO), 1980, pp. 429-436. 

12. Burrows, J. (ed.) Environmental Engineering in South African Mines. 
Mine Vent. Soc. South Africa, Cape and Transvaal Printers (Pty) Ltd., Cape 
Town, South Africa, 1982, p. 884. 



2S 



DIESEL-DISCRIMINATING FIRE SENSOR 



By Charles D. Litton 1 



ABSTRACT 

This Bureau of Mines paper describes a novel fire detector that can be used to 
discriminate between smoke produced by a fire and smoke produced by a diesel engine. The 
detector uses a pyrolysis technique whereby a sample of smoke-laden gas passes through a 
short, heated tube within which fire smoke particles pyrolyze and increase their number 
concentration and decrease their average size, while diesel smoke particles are unaffected. 
The detector is designed for use in mines that use diesel-powered equipment where the 
detection of fires is complicated because of the diesel emissions background levels of smoke 
and other products of combustion. 



BACKGROUND 



Diesels produce smoke, CO, CO z , and other combustion 
products. In an underground mine that uses diesel-powered 
equipment, these diesel exhaust products can mix with the 
ventilation airflow resulting in concentration levels sufficient 
to produce frequent false alarms of product-of-combustion 
fire sensors, such as smoke and CO sensors. Further, if a fire 
were to occur in the presence of these elevated product levels, 
its detection could go unheeded, reaching a substantial size 
before it is finally detected. As a result, early warning capa- 
bilities (crucial to life safety) are seriously compromised. The 
problems of false alarms and degradation of early warning 
capabilities in diesel-operated mines require that some type of 
fire sensor be used that essentially ignores the diesel back- 
ground levels yet remains very sensitive to combustion prod- 
ucts produced by fires. 

Previous contract research by the Bureau of Mines 2 
identified the ability of fire smoke particles to pyrolyze upon 
passage through a short, heated tube whose surface tempera- 
ture was held constant between 300° and 350° C. Upon 
passage through this pyrolysis tube, the number concentra- 
tions, n , of fire smoke particles increased dramatically. At the 
same time there was a corresponding reduction in the number 
mean particle diameter, d . Diesel smoke particles were unaf- 
fected upon passage through the same pyrolysis tube. 



1 Supervisory physical scientist, Pittsburgh Research Center, Bureau of Mines, 
Pittsburgh, PA 

2 Skala, G. F., and F. W. Vanluik, Jr. Development of Selective Submicrometer 
Particulate Fire Detectors for Underground Metal Mines (contract H0387O25, 
Environment/One Corp.). BuMines OFR 58(1 )-83, 1979, 129 pp.; NT1S PB 
83-178947. 



A prototype detector was subsequently built based upon 
this pyrolysis principle. The incoming smoke-laden gas was 
split into two parallel paths, one path containing the pyrolysis 
tube, the other path being a straight section of tubing. The 
concentration of smoke along each path was measured by a 
cloud condensation nuclei counter and an alarm threshold for 
fire set at a ratio of pyrolyzed to unpyrolyzed concentrations 
equal to 1.5. Even though the prototype functioned as ex- 
pected, it was not well suited to the mine environment. 

At about this same time, Bureau researchers had devel- 
oped and patented a sensitive smoke detector for use under- 
ground. This sensor uses an ionization chamber to efficiently 
charge smoke particles and measure their concentration. In 
August 1983, the U.S. Patent Office granted an exclusive 
license to a major manufacturer of mine monitoring equip- 
ment to develop and market this detector commercially. This 
detector's response is a function of the size and concentration 
of the smoke particles and its principles of operation have been 
discussed in detail elsewhere. 3 Basically, the response increases 
with increasing smoke particle diameter and concentration. 
Because the pyrolysis tube increases the fire smoke concentra- 
tion but decreases the average particle diameter, could this 
sensor be used to measure both pyrolyzed and unpyrolyzed 
smoke particles and if so, would it be sensitive enough to use 
as a discriminating, early warning fire sensor in diesel- 
operated mines. A series of laboratory experiments were then 
conducted to answer this question. 



3 Litton, C. D., L. Graybeal, and M. Hertzberg. Submicrometer Particle 
Detector and Size Analyzer. Rev. Sci. Instrum., v. 50, No. 7, July 1979, pp. 
817-823. 



29 



LABORATORY EXPERIMENTS 



The system used to conduct the initial laboratory experi- 
ments is shown in figure 1 . It consisted of a cubical chamber, 
45 cm on each edge, and constructed of plexiglass. Exhaust 
from a diesel-operated generator could be directed into the 
chamber via a short tube. Inside the chamber, a hotplate was 
used to heat samples of mine combustibles. Directly above the 
hotplate, a sample tube continuously pulled gas samples from 
the chamber for analysis by the Bureau smoke detector. 
Outside the chamber, this sample tube split into two parallel 
paths; with one path of flow containing the pyrolysis tube and 
the second path, a plain, unheated section of tubing. Flow was 
diverted along one path or the other by turning valve A or B. 
In this manner, the smoke detector could sample the unpyro- 
lyzed smoke particles and the pyrolyzed smoke particles. The 



pyrolysis tube was a 2.8-cm-long piece of 0.64-cm-diam stain- 
less steel tubing heated resistively to a surface temperature of 
350° C. 

Combustible samples of coal, wood, conveyor belt, and 
plastic line brattice were tested both in the presence of diesel 
smoke and without diesel smoke. Measurements of diesel 
smoke alone were also made during these series of experi- 
ments. In a typical test, the combustible material was heated to 
a stage of sustained smoldering and then the unpyrolyzed and 
pyrolyzed signals measured alternately by switching valves A 
and B. With tests in the presence of diesel smoke, a steady- 
state diesel smoke level was established and the combustible 
material was then heated in the same manner and sampled 
alternately along each path. 



LABORATORY STUDIES 



These tests verified that diesel smoke showed no effects of 
pyrolysis. Various diesel smoke levels through the pyrolysis 
path and through the unpyrolyzed path were constant, indi- 
cating that diesel smoke showed no pyrolysis behavior. 

In the experiments, the particle sizing capabilities of the 
smoke detector were used to determine the average diameters 
of the smoke particles. For diesel smoke, the average diameter 
was found to be 0.23 ^jm. For the fire smoke, the average 
diameters varied from -0.05 to -0.70 /xm. 

Fire smoke particles produced in the absence of diesel 
smoke were found to pyrolyze linearly with their average 
diameter, d . A plot of the data obtained for the four 
combustible materials tested is shown in figure 2, where G n 
represents the ratio of number concentrations of the particle 
leaving the pyrolysis tube, n' Q , to the number concentrations 
entering the pyrolysis tube, n . That is, G„ o = n' /n . A linear 
curve fit of this data yielded 



Unpyrolyzed flow 



n'/m = G„ = 55-d„ 



(1) 



where d = initial unpyrolyzed fire smoke particle diameter, 
nm. Further, the average particle diameter, d' , of the repyro- 
lyzed smoke particles was found to vary according to 



d'„ = 0.27 d 2/3 , 



(2) 



with d' Q and d in micrometers. 

To summarize, the larger the average fire smoke particle 
diameter, d , the greater is the increase in number concentra- 
tion upon passage through the pyrolysis tube. The pyrolyzed 
smoke particles have a smaller average particle diameter, but 
the increase in concentration is sufficiently great that the 
effects of particle size are negligible. 

Now, when a combination of fire smoke particles and 
diesel smoke particles are present, only the fire smoke particles 
pyrolyze. If the concentration of fire smoke particles is low 
relative to the concentration of diesel smoke particles, the ratio 
of pyrolyzed to unpyrolyzed signals will be low. At the other 
extreme, if the concentration of fire smoke particles is high 
relative to the concentration of diesel smoke particles, this 
ratio will be higher and will approach the value obtained if no 
diesel smoke is present. However, the difference between 
pyrolyzed and unpyrolyzed signals is not significantly affected 
by the concentration of diesel smoke particles since it is the fire 
smoke particles only that pyrolyze. 



Sample tube 



Exhaust 




Pyrolyzed & 




Diesel 
generator 



\t{— — Hotplate 

9-m 3 with sam P |e 
chamber 



Pyrolysis 
tube 



Figure 1 .—Experimental setup for measuring response char- 
acteristics of Bureau's detector to smoke particles passing 
through a small pyrolysis chamber. 



'no 



60 
40 

20 

IO 
8 

6 
4 



i — < — i — ' i ' i 1 1 1 1 — i — i — i— r 




j i 



KEY 
o Wood smoke 
• Coal smoke 
■ SBR conveyor 

belt smoke 
a PVC line brattice 

smoke 

_l I I I I L 



0O2 



004 006 008 0.10 



0.20 



0.40 0.60 0.80 1.00 



do, /Am 



Figure 2. — Ratio of number concentration of smoke particles 
pyrolyzed to number concentration entering the repyrolysis 
element as a function of the average diameter of the particles 
entering the pyrolysis tube. 



30 



These laboratory results indicate that a pyrolysis detector 
using the Bureau smoke detector as the primary sensor and 
utilizing the difference between pyrolyzed and unpyrolyzed 



signals to distinguish between fire smoke and diesel smoke has 
the potential to reliably detect fires in the presence of signifi- 
cant diesel background levels. 



PYROLYSIS DETECTOR 



The use of the pyrolysis concept requires that both a 
pyrolyzed and an unpyrolyzed signal be measured. One option is 
to devote a separate detector to each sampling line. This option, 
however, requires not only two sensors, but two sensors whose 
response characteristics are very similar (ideally, identical). 

Now, the conventional Bureau smoke detector utilizes an 
ionization chamber consisting of a set of parallel plate elec- 
trodes with one electrode having 5 piCi of americium 241 
deposited uniformly on its surface. To create two identical 
chambers, a piece of fluorocarbon polymer, 1/8-in-thick, was 
used to divide the original chamber into two separate, distinct 
chambers. Each of these new chambers has its own electrically 
isolated negative electrode, while the positive, radioactive 
electrode is shared by the two chambers so that only one 
common voltage source is needed to power both chambers. 
The result is two identical, independent ionization chambers, 
one chamber continuously measuring the pyrolyzed smoke 
particles while the other measures the unpyrolyzed smoke 
particles. 



The original pyrolysis tube required ~ 45 A at a voltage of 
0.27 V to maintain the 300° C temperatures necessary for 
operation. To reduce the current requirements for the pyrolysis 
tube, a new tube was fabricated which utilizes a 2.0-in-long, 
1/32-in-diam ceramic rod. Nichrome wire of 0.008 in diameter 
and ~9.0 in long is wound around the ceramic rod and the 
ends of the nichrome electrically connected by a 6-V power 
supply which consumes -0.70 A. This rod is then inserted 
into the air space between two swagelock fittings. At the gas 
inlet port to the new sensor, a T-connection allows one-half of 
the flow to pass through this new pyrolysis tube to its 
ionization chamber while the other half of the flow passes 
through a plain section of tubing to the other ionization 
chamber. 

Figure 3fi shows the new pyrolysis tube connected to the 
inlet of one of the ionization chambers. Figure 3A shows the 
dual ionization chamber with an attached pyrolysis tube 
mounted in a housing with the electronics, internal pump, and 
other necessary components. 



PROTOTYPE OPERATION 



When no smoke particles are present in the air being 
sampled, the output signals of both chambers are amplified 
and electronically adjusted to identical levels of ~ 7.0 V. When 
smoke enters either chamber, the signal levels decrease accord- 
ing to the expression 



where 
V c 

d 
"o 



V = (V /k d n )(l -exp(-k d nj), 



(3) 



steady state chamber signal in the absence of 

smoke, ~7.0 V; 

smoke particle diameter, cm; 

smoke particle concentration, p/cm 3 , 

chamber constant, 0.0025 cm 2 /p. 



and 



For example, at an average smoke particle diameter of 
d = 0.20 pim (2 X 10" 5 cm), a 1-pct reduction in unpyrolyzed 
signal occurs when the smoke concentration is 4 x 10 5 p/cm 3 . 
However, from equations 1 and 2, the value of n' when d is 
0.2 ftm is n' D = 4.4 x 10 6 and d' Q is 0.092 ^m (9.2 X 10" 6 cm) 
so that d' -n' is 40.6 and the corresponding signal reduction 
in the pyrolysis chamber is — 5 pet. This means that the 
unpyrolyzed signal minus the pyrolysis signal is -0.28 V. 
Subsequent data acquired with the prototype indicates that an 
alarm threshold around 0.3 to 0.5 V appears reasonable. 






* # 




Figure 3.— Photographs of the major components of the pyrolysis fire detector. A, Dual ionization chamber; 8, new 
pyrolysis tube connected to inlet of one ionization chamber. 



31 



PROTOTYPE EVALUATION 



The prototype pyrolysis fire detector was subsequently 
evaluated in a series of intermediate-scale tests. For these tests, 
a diesel exhaust was diverted, via a 4-in-diam flexible hose, 
into a ventilated intermediate-scale fire tunnel. 4 The resultant 
difference signal due to diesel smoke levels was measured with 
the detector prototype. 

After a steady-state diesel level has been established, 
approximately 9 to 10 kg of coal was heated with an imbedded 
electrical strip heater, producing both smoldering and eventual 
flaming combustion of the coal. Figure 4 is typical of the 
relative increases in both pyrolysis difference voltage and part 
per million CO, as the coal mass begins to smolder. 

It is worth noting that the difference in signal due to diesel 
operations only is less than the zero level signal established due 
to only ambient air while the diesel-only CO level is 115 ppm. 
When the CO has increased to 15 ppm above this ambient 
level, the pyrolysis detector signal is 1.60 V. 

In another test, the coal fire was allowed to develop 
without any diesel background. The data are shown in figure 
5. The results of this test are essentially the same. At 19 min 
into the test, the diesel was turned on and the CO level 
increased from 75 ppm to 230 ppm while the pyrolysis detector 
was virtually unaffected. 

Several subsequent intermediate-scale tests have been con- 
ducted to determine the response of the pyrolysis detector to 



4 Egan, M. R. Transformer Fluid Fires in a Ventilated Tunnel. BuMines IC 
9117, 1986, 13 pp. 




10 15 
TIME, min 

Figure 4.— Response of a CO detector (A) and the pyrolysis 
fire detector (B) to a developing fire in the presence of diesel 
exhaust combustion products. 



smoldering combustibles. Table 1 shows the data obtained 
during steady-state smoldering at combustible surface temper- 
atures of -300° C. 



Table 1 . — Average pyrolysis difference voltages, 

smoke diameters, and concentrations obtained for 

smoldering combustion 

Sample v"V, V d , nm n c , p/cm 3 

Styrene butadiene rubber belt 0.45 0.14 2.0 x 10 6 

Neoprene belt 2.52 .52 8.5 x 10 s 

PVC belt 1.05 .31 7.5 x 10 5 

Plywood 3.30 .30 2.2 x 10 6 

Coal 2.50 .35 1.3 x10 6 



225 


i i 
_ A 


■ i ■ i ■ i 


1 


200 


- 




' - 


I75 


- 




- 


E I50 


— 


Diesel | 


- 


Q. 

t I25 
o 




an 
flan- 
First 


i 
ie / 


- 


° I00 




visible 

smoke name 






75 






15-ppm 








50 






CO alarm 






' 


25 














- 



2.6 


■ 1 1 i 






i i 


i 


i 


i 














1 


1 


1 


i i 


i 


fJ \ . 


i 


2.3 


_ B 








J 


V 


V*- 


> 








/ 


u/V 






- 2.0 


- 






/ 


rV r 




— 


_i 








r 








< 1 .7 


— 






J 






— 


z 








•r 








<2 |. 4 


— 






/ 






- 


(/) 








' 








UJ I.I 


- 












- 


o 
















2 .8 














— 


UJ 
















UJ 3 


- 












- 


U_ 
















u- .2 


- 












— 


-.1 


^^ 












_ 


— a 


i 


i i 


n i 


i i \ 


N , 


M 


i 







10 15 

TIME, min 



20 25 



Figure 5.— Response of a CO detector (A) and the pyrolysis 
fire detector (B) to a developing fire when no diesel combustion 
products are present. 



32 



CONCLUSIONS AND DISCUSSION 

The concept of repyrolysis of fire smoke particles appears thus reducing, if not totally eliminating, the problems of false 

to be a valid concept for rapid and reliable detection of fires in alarms of fire sensors due to diesel-produced combustion 

mines using diesel-powered equipment. The low pyrolysis products. Initial tests to evaluate the performance of a proto- 

temperatures have no positive effect on diesel smoke particles, type pyrolysis fire detector have been very encouraging. 



33 



COMPUTER-AIDED MINE FIRE SENSOR DATA 
INTERPRETATION IN REAL TIME 



By L.W. Laage, 1 W.H. Pomroy, 2 and A.M. Bartholomew 3 



ABSTRACT 

Throughout recorded history, underground fires have plagued mining operations. Compared 
with other hazardous situations in underground mining, a fire can become a global problem by 
swiftly spreading deadly carbon monoxide and other products of combustion (POC's) throughout 
the whole mine, often without warning. Experience has shown that when fires are detected and 
located in their early stages, they are much easier to control and proper escape routes can be more 
intelligently selected. Recent advances in sensor and data communication technology have made 
reliable mine fire detection system installations possible. Unlike building construction, in a mine, 
it is impractical to install detectors at every desired location. Abandoned workings and unsafe loca- 
tions preclude sensor installations from both safety and economic standpoints. The net effect is 
that some fires are detected and located early while the location of others, even if detected early, 
remains unknown too long for effective evacuation and fire fighting. 

This paper discusses recent research by the Bureau of Mines to develop a strategy to locate 
mine fires in real time using a minimum of selectively placed sensors coupled with computer-aided 
data interpretation. 



INTRODUCTION 



Underground mining continues to rank among the most hazard- 
ous of all industrial occupations (I). 4 This disparity is particularly 
evident in the case of fires. During 1984 the incidence rate for in- 
dustrial fires in the United States, expressed as the number of fires 
per 200,000 worker-hours, was 0.095 (2). During the same year, 



'Mining engineer. 

Hjroup supervisor. 

'Statistical assistant. 
Twin Cities Research Center, Bureau of Mines, Minneapolis, MN. 

'Italic numbers in parentheses refer to items in the list of references at the end of 
this paper. 



the fire incidence rate for underground metal and nonmetal mines 
was 0. 15 1 ; 60 pet higher than for general industry (3). The poten- 
tial for disaster is compounded by the limited number of possible 
escape routes and limited fresh air supplies available underground. 
In the event of a fire, fresh air supplies and safe travelways are 
actually decreasing. 

This paper demonstrates a computer simulation technique for 
locating underground mine fires utilizing an array of strategically 
placed underground fire detectors. This technique provides an 
automated means by which the network branch, or set of network 
branches terminating in a single junction, in which a mine fire occurs 
can be determined quickly and with reasonable confidence. 



34 



ACKNOWLEDGMENT 



The authors wish to acknowledge the assistance of John R. 
Marks, chief ventilation and health engineer, Homestake Mining 
Co., for providing detailed ventilation data for the Homestake Mine 



in Lead, SD, which was the subject of the case study experiment 
discussed in this paper. 



BACKGROUND 



Rapid determination of the location of an underground mine 
fire has long been the dream of mining personnel. If the location 
is known, preferred escape routes can be designated, rescue teams 
can concentrate searches in areas of greatest need, and firefighters 
can select the most efficient route to the fire and the most effective 
fire-fighting strategy. Also, once the location is known, other related 
aspects of the fire such as intensity, fuel, and growth rate can be 
inferred. 

Occasionally, fires occur at an active working face, shop, or 
other area that is under direct observation. In these cases, immediate 
action can be taken to notify appropriate mine officials regarding 
the fire's location and other relevant information. However over 
one-half of the fires in underground metal and nonmetal mines 
reported to the Mine Safety and Health Administration occur in 
inactive or otherwise unoccupied areas (i). Miners eventually see 
or smell the smoke, however it is impossible for them to know, 
without further investigation, its origin. 

Fire bossing (the systematic inspection of mine workings for 
fire) is the only means available for determining the location of an 
underground mine fire. The shortcomings of fire bossing are 
threefold: it is slow (especially in large mines); inspectors are subject 
to severe fire, smoke, and toxic gas hazards; and, ironically, it is 
often unsuccessful in determining the exact location of a fire. The 
latter occurs because progress of the fire boss is stopped if heavy 
smoke is encountered. 

Fire bossing is more effective in mines equipped with 
strategically located electronic fire detection devices, as the search 
for fire can be restricted to the general area of the detector(s) in 
alarm. Over the past 15 yr, use of electronic fire detection devices 
in underground mines has expanded from a few research installa- 
tions to become an accepted industry practice (4). Systems employ- 
ing 40 or more monitoring points have been installed in some mines, 
with coverage ratios (ratio of number of detectors to number of 
network branches in a mine) typically ranging from 1:50 to 1:10. 

However, even in mines equipped with fire detectors, personnel 
must still be relied upon to locate the fire. The shortcomings of 
fire bossing are somewhat mitigated by the use of fire detection 
devices; however, the overall effectiveness of fire bossing is 
inherently limited. 

The simplest solution to the problem of quickly and safely 
locating a fire's source is to install a fire detector in every network 
branch in the mine. However as most mines comprise hundreds 
of branches, installation of a detector in every branch would be 
cost prohibitive. 

A more practical approach involves integration of a fire detec- 
tion system with a ventilation network analysis computer model. 
In the event of fire, POC's flow along predictable courses in a mine, 
following the ventilation. It is hypothesized that by carefully noting 
the pattern and timing of alarms from a few strategically placed 
detectors with reference to known ventilation flows, it should be 
possible to determine the network branch in which a fire is located. 
Although fires can themselves alter ventilation flow conditions, thus 
interfering with such a determination, it is further hypothesized that 



during the early stages of a fire the initial distribution of POC's 
would precede ventilation disturbances. 

Because fires in their initial stages typically produce copious 
amounts of smoke and CO but generate little heat to disturb the 
ventilation flow, this second hypothesis was considered reasonable. 
However, for further support, a study of the data base of reported 
metal and nonmetal underground mine fires from 1950 to 1984 was 
conducted to determine, where possible, whether this slow grow- 
ing fire scenario was valid (3). From a practical standpoint, only 
fires not immediately detected would be located with this strategy. 
This strategy would apply to slow growing fires with modest flame 
spread rates, such as burning timber, insulation, or conveyor belting. 
Sites with a potential for fast growing fires with high flame spread 
rates, such as fuel storage locations, would require individual 
detectors. 

The method of fire detection was divided into three categories: 
(1) Worker not in immediate area, incoming shift workers, and shift 
boss, foreman; (2) operator or worker in immediate area and 
welding crew; and (3) other and unspecified. In category 1 there 
. were 104 reported underground fires, category 2 had 81, and there 
were 44 in category 3. Category 1, where the fire was not im- 
mediately detected, was then analyzed with respect to equipment, 
ignition source, burning substance, and location. 

Equipment: Excluding the category not specified, electrical 
equipment incidents made up the largest group, with 23, followed 
by mobile equipment with 12, and conveyors with 9 incidents. Of 
the 23 electrical equipment incidents, 18 had electrical as the igni- 
tion source. 

Ignition source: Fire incidents caused by electrical ignition 
sources accounted for 41 of the reported underground fires; spon- 
taneous combustion followed with 17 and welding with 14. Not 
surprising is that spontaneous combustion was the cause of 10 out 
of 17 fires in the mined-out and/or waste location. 

Burning substance: The burning substance involved in the 
majority (55 pet) of the reported underground fires was timber. 
There were 24 fire incidents that involved two burning substances, 
mainly insulation and the category other. Three of the fire incidents 
involved three burning substances: insulation, rubber, and other. 
When comparing burning substance with location, of the 20 shaft- 
raise- winze location fires, 18 of them reportedly had timber as the 
burning substance. 

Location: The haulage-drift area had the highest number of fire 
incidents with 34. The majority of the welding fires occurred in 
the haulage-drift and shaft-raise-winze locations. Fourteen of the 
eighteen fires in the substation-shop-storage-pump location were 
caused by electrical equipment. 

Eighty percent of the fires involved timber, insulation, or rub- 
ber while twelve percent involved unknown materials. Only 8 pet 
of the fires involved combustible liquids. 

From this study it appears that most of the fires that were not 
immediately detected in metal and nonmetal mines started small 
and/or had been slow growing in nature with little potential for ven- 
tilation disturbances in the early stages of the fire. 



35 



DESCRIPTION OF FIRE LOCATION ALGORITHM 



Based on the two hypotheses, an algorithm was developed to 
utilize the real-time outputs from a system of strategically placed 
underground detectors as inputs to a computer model for deteimining 
the location of an underground mine fire. The algorithm involves 
three steps, as follows: 

1. Instrumentation.— Electronic instrumentation is installed at 
strategic underground locations. Environmental parameters to be 
monitored include POC's such as CO and C0 2 , air temperature 
velocity and direction, and barometric pressure. The number and 
placement of POC detectors is critical and is determined iteratively. 
The detectors are linked through appropriate telemetry to a master 
control. 

2. Computer Simulation.— In the event of fire underground, 
the current state of the mine ventilation is simulated using mine 
ventilation network analysis software (5). The ventilation monitoring 
instruments (temperature, velocity, direction, and pressure) pro- 
vide real-time input data to the network analysis program, thereby 
insuring a high degree of accuracy. Potential fire locations are deter- 
mined through an analytic procedure that identifies all upstream 
network branches common to the detectors in alarm. As POC waves 
reach additional detectors, potential fire locations are redefined 
through a "tree walk" procedure (i.e., impossible locations are 
eliminated). Ventilation travel times are then calculated from every 
network branch that is a potential fire location to every network 
branch in which a fire detector is located. The travel times are deter- 
mined using average air velocities and branch lengths. Next, 
predicted combustion product arrival times (CPAT's) are calculated 
from every potential fire location to each detector. CPAT is the 
elapsed time beginning with the arrival of POC's at the first alarm- 
ing detector (defined as arrival time=0 for that detector) until subse- 
quent detectors begin to alarm. For example, if POC's reach detector 
A first, detector B 3.5 min later and detector C 2.6 min after that, 
the respective CPAT's would be for A, 3.5 for B, and 6.1 for 
C. The CPAT's are then stored in a data array. 

3 . Fire Location Determination. — The computer engages in an 
array scanning routine wherein the real-time pattern of incoming 
detector alarms is compared with the CPAT's stored in the data 
array. The data array fire location corresponding to the combus- 
tion product arrival time pattern that best matches the pattern of 
real-time incoming detector alarms is the most probable location 
of the actual fire. 



The iterative process for determining proper detector number 
and placement involves three steps. First, initial detector locations 
are chosen. These initial locations can be based on somewhat 
arbitrary criteria, such as the availability of power or site accessi- 
bility. Next, the data array is created through computer modeling. 
Finally, this data array is scanned to identify entries having similar 
arrival time patterns. Adjustments to the number and placement 
of detectors are made and the last two steps repeated until no similar 
arrival time patterns are produced. Through this process, potential 
fire locations can be associated with a recognizable pattern of 
CPAT's at the various detector locations. 

At this juncture, it is important to note a characteristic of the 
fire location algorithm which, under certain circumstances, may 
make it impossible to distinguish between two or more network 
branches that have equal possibility of being the fire's true loca- 
tion. A necessary condition for the proper function of the algorithm 
is that the branch in which the fire is located terminate at a junc- 
tion that splits the air into two independent paths leading to respec- 
tive detectors. At junctions where separate splits of air merge, each 
split loses its individual identity (perfect mixing at the junction is 
assumed). A downstream detector can not determine from which 
split the combustion products originated. 

As a result of this characteristic, the algorithm is unable to 
resolve fire location in two situations. The first situation is where 
two or more branches enter a junction and two or more branches 
exit the same junction. If detectors are appropriately located in 
independent paths downstream of the junction, the algorithm would 
indicate equal probability of the fire occurring in each branch enter- 
ing the junction. This outcome still represents a significant improve- 
ment over fire bossing, however, as the search could be limited 
to those branches entering the junction. 

The second situation occurs where two or more branches com- 
bine at a junction to produce a single branch. In this situation, the 
condition requiring two independent paths to respective detectors 
cannot be satisfied, hence the fire's location remains uncertain. 
However this outcome would be limited to a relatively small number 
of branches on the return side of the ventilation system and would 
therefore not significantly diminish the overall value of the 
algorithm. 



FIRE LOCATION CASE STUDY EXPERIMENT 



Using the fire location algorithm, a hypothetical case study 
experiment was performed. The objective of the experiment was 
to determine whether detector locations could be selected that would 
produce a recognizable pattern of CPAT for every potential fire 
location and to determine if a slow growing fire would adversely 
affect the performance of the differential arrival time algorithm. 
In order to test the algorithm under as realistic circumstances as 
possible, the subject of the experiment was a portion of the ven- 
tilation network from the Homestake gold mine in Lead, SD. 

The Homestake Mine is a complex multilevel mine having in 
excess of 2,000 airways. By combining parallel airways, a simplified 
ventilation network consisting of 404 branches and 237 junctions 
was created. The case study experiment was performed on a sec- 
tion of the mine comprising 83 branches and 48 junctions. A 



simplified illustration of the overall ventilation network and the case 
study test section are shown in figure 1. 

The experiment began by applying conventional nework 
analysis techniques to determine airflows, velocities, and resulting 
branch times (time required for air to travel the entire length of 
a branch). Next, locations for fire detectors were specified. The 
hypothetical detection system initially comprised seven detectors, 
resulting in a coverage ratio of 1:11.9. The test section, airflow 
directions, branch times, and initial detector locations are shown 
in figure 2. CPAT's were then computed from every branch in the 
test section (i.e., potential fire location) to each detector location. 

This procedure produced three distinct zones within the test 
section (fig. 3). In zone I, fire location was resolved to a single 
branch or pair or adjoining branches, indicating satisfactory detector 



36 



placement. In zone II, three groups of 8 to 10 branches each had 
the same arrival time patterns, indicating the need to either relocate 
detectors, add detectors, or both. In zone III, the independent paths 
to respective detector locations, which are a necessary condition 
for proper operation of the fire location algorithm, led outside the 
test section and hence, fire location could not be resolved to a single 
branch or pair of adjacent branches. However, appropriately located 
detectors outside the test section would provide satisfactory resolu- 
tion of fire location in these branches. 

Zone II was subject to further analysis to improve resolution 
of fire location in that zone. Examination of arrival times indicated 
that two of the detector locations, B and F, did not uniquely identify 
any fire location and could thus be eliminated from the detection 
system with no adverse effect. Detectors B and F were relocated 
to positions H and I. An eighth detector, J, was also added. The 
detection system now comprised eight detectors, resulting in a 
coverage ratio of 1:10.4. 

Using the new detector locations, CPAT's were again com- 
puted. Satisfactory resolution was achieved for 22 of the 27 branches 
composing zone II. Fire location uncertainty remained for the five 
branches terminating at junctions 69, 70, and 71. However the 
condition requiring at least two independent paths to respective 
detector locations could not be satisfied for these branches, hence 



the detector location study was concluded. If resolution of fire loca- 
tion in these branches was desired, additional detectors could be 
installed as needed. Initial and final detector locations, as well as 
the remaining area of fire location uncertainty are shown in figure 4. 

A fire was then added at the start of the branch between junc- 
tions 46 and 65 to evaluate the effect of a slow growing fire on 
the differential arrival time for that airway. To further complicate 
the problem, the environmental conditions in the mine were changed 
from completely saturated air at 60 ° F to 9 pet relative humidity 
air at 75° F. 

The arrival times were recalculated with a ventilation simulator 
MFIRE (6) capable of dynamic simulation of transient state ven- 
tilation under the influence of mine fires. Figure 5 shows the new 
POC travel times in the case study area under the changed condi- 
tions before initiation of the fire. POC travel time to detector H 
was 10.65 min and travel time to detector I was 13.88 min under 
these initial conditions. 

After the initiation of a 1 ,000 Btu/min fire (the size of a trash 
can fire), with dynamic updates to the state of the ventilation system 
every minute, the POC spread was again tracked to detectors H 
and I. New arrival times were 10.70 min to detector H and 13.93 
min to detector I, a very close agreement. 




Case study 
test section 



Figure 1 .—Mine ventilation network with case study test section indicated. 



37 




Junction 

00.0 Branch time, min 
— ^ Airflow direction 
Network branch 



^ Branch exiting test section 
Initial detector location 



Figure 2.— Case study test section showing airflow directions, branch times, and initial detector locations. 



38 



1.3 



1.8 




1.0 



1.3 




2.0 



2.6 



3.7 



2.2 



1.8 



4.8 



1.6 



^^ 



.T^ 



0.4 



(109)- 
O.2] 
(l08> 



0.2 
(l07V 



0.3 



0.4 
— (l05> 



0.6 
— (104> 



0.8 
fl03> 



1.5 



102} 



KEY 
Junction 

00.0 Branch time, min 
wm^m Branch comprising zone 

.•.•.•.•.■.v.-. Branch comprising zone 
Branch comprising zone 



Figure 3. — Case study test section showing zones of varying fire location capability with detectors in initial locations. 



39 




00.0 Branch time, mln 
Ml New detector locations 

Initial detector locations 

Remaining area of fire 
location uncertainty 

Network branch 



Figure 4.— Case study test section with initial and final detector locations and remaining area of fire location uncertainty. 



40 



A 



/\ 



.20 



.22 



.22 



.24 



.29 



.29 



.33 



.36 



.85 



14.30 



6.30 



11.52 



11.54 



9.45 



150.00 



8.52 



9.27 



7.73 




56 



i.16 



—(57 



4.15 

5fT 



4.49 



59 



[41.67 
[60 

H LJ 1. 
61 



1.32 



62 



0.58 



63 



0.55 



64 



0.47 



65 



9.84 



6.83 



7.82 



9.11 



4.79 



2.78 



3.85 



45.61 




0.90 



1.93 



2.48 



3.46 



2.16 



1.77 



4.63 



1.50 



/\ 



0.42 



(109 



0.18 
{108) — 



E h 0.21 
-0) 



[0.30 
{106V 



t X" 42 

— (fosV- 



[0.57 
-(104) 

J 0.80 

{uSV - 



1.28 
{102) := 



1.90 



KEY 

00.0 Travel times, min 

I— I Detector location 

00 ' Junction 

► Airflow direction 

^. Branch exiting test section 

_ Network branch 

■ Product of combustion path 



(1 Fire 



Figure 5.— Case study test section showing new conditions and POC paths to detectors. 



41 



DISCUSSION OF EXPERIMENT RESULTS 



The case study experiment demonstrates the operation of the 
fire location algorithm. Several observations relating to the results 
of this case study experiment are discussed in the following 
paragraphs. 

This test illustrates the need to maintain a high degree of control 
over mine ventilation parameters for the fire location system to 
perform properly. In some instances, the basis for specifying one 
network branch over another as the probable fire location was a 
difference in CPAT's of less than 0.5 min at a single detector. If 
knowledge of ventilation flow parameters is only approximate, the 
resulting inaccuracies would preclude a proper determination of fire 
location. 

This test illustrates the importance of proper detector place- 
ment. With detectors in the initial seven locations, a large blind 
area was produced where the location of the fire could not be 
specified. Only after detectors B and F were moved (to locations 
H and I) and detector J added, was the system capable of performing 
properly. In larger and more complex networks, several detector 
relocation iterations could be necessary. It should also be noted that 
if mine conditions (no power, inaccessibility, etc.) prevented place- 
ment of detectors in the required locations, a system performance 
degradation could be expected. 

This test illustrates the need for a high level of detection system 
reliability for the fire location function to perform properly. The 
loss of one or two detectors (power outage, telemetry failure, zero 
or span drift, fuel cell failure, etc.) would seriously compromise 
the ability of the system to determine the correct fire location. In 
addition to systematic detector and telemetry inspection and 



maintenance, consideration should be given to installing multiple 
detectors at each detector outstation to provide a backup detection 
capability in case of detector failure. 

The algorithm can perform well for small fires in their initial 
stage. 

The need for mine ventilation parameter monitoring is essential 
to obtain an accurate version of the current state of the ventilation 
system. This is illustrated by casual inspection of the variations of 
travel times between figure 4 and figure 5. 

Certain enhancements to the system are possible that could 
potentially improve its speed and accuracy. One obvious enhance- 
ment is the inclusion of POC concentrations in the fire location 
scheme. If the pattern of POC concentration was tracked, the ar- 
rival of second and third waves of POC's (air that had originated 
at the fire site and traveled to the detector over well-defined but 
slower paths than that accounting for the first arrival) could be noted. 
The pattern of second and third arrivals, observed at a single detector 
or at multiple detector locations, could help define a unique fire 
location where the same detectors, using first arrival times alone, 
would leave blind areas. 

The authors also wish to emphasize that a single experiment 
consisting of simulations performed on data from a portion of a 
single mine hardly constitutes a definitive treatment of this sub- 
ject. Additional simulations to exercise the algorithm over a broad 
range of mine types and ventilation conditions are planned. Because 
the fire location system incorporates certain assumptions regarding 
both fire and ventilation system behavior, full-scale in-mine valida- 
tion tests are also planned. 



CONCLUSIONS 



A technique for determining the location of an underground 
mine fire using a system of underground fire detectors and a spe- 
cially designed ventilation network analysis computer model has 
been presented. The technique is simple and straightforward and 
can be implemented using off-the-shelf, mineworthy detection, 
telemetry, and computer hardware. The technique has certain limita- 
tions relating to ventilation network configuration; however, the 
limitations affect only small parts of a network, and thus do not 
detract significantly from its many potential benefits. 

Although the findings of the case study were favorable, the 
reader should be cautioned that this approach to mine ventilation 



and fire safety analysis is meant to supplement, and not supplant, 
the traditional decisionmaking processes at a mine. This system 
should be regarded as a source of heretofore unavailable informa- 
tion which, when integrated with other relevant data, can form the 
basis for important ventilation decisions. It should not be used as 
the sole data source for an automated closed-loop feedback system 
of ventilation controls. The complexity of modern mine ventilation 
systems and the compounding effect of the sometimes unpredictable 
human element dictate that such controls be exercised only by teams 
of trained and experienced experts. The value of the fire location 
system is in the information it supplies to human decisionmakers. 






REFERENCES 



1. Stout-Wiegand, N. National Traumatic Occupational Fatalities, 
1980-1984. NIOSH, Div. Saf. Res., Morgantown, WV, June 11, 1987, 
12 pp. 

2. Cote, A.E. (ed.). Fire Protection Handbook. National Fire Protec- 
tion Association, Quincy, MA, 16th ed., 1986, p. 1-9.3. 

3. Butani, S.J., and W.H. Pomroy. A Statistical Analysis of Metal and 
Nonmetal Mine Fire Incidents in the United States From 1950 to 1984. 
BuMines IC 9132, 1987, 41 pp. 

4. Marks, J.R. Carbon Monoxide Monitoring at the Homestake Gold 
Mine — Lead, South Dakota. Pres. at Am. Min. Congr. Min. Convention, 



San Francisco, CA, Sept. 13-16, 1987, 12 pp.; available upon request from 
L.W. Laage, BuMines, Minneapolis, MN. 

5. Edwards, J.C., and R.E. Greuer. Real-Time Calculation of Product- 
of-Combustion Spread in a Multilevel Mine. BuMines IC 8901, 1982, 
117 pp. 

6. Chang, X. The Transient-State Simulation of Mine Ventilation Systems. 
Ph. D. Thesis, MI Technol. Univ., Houghton MI, 1987, 162 pp. 



42 



RELIABILITY OF UNDERGROUND MINE FIRE DETECTION 
AND SUPPRESSION SYSTEMS 



By Steven G. Grannes 1 



ABSTRACT 

The Bureau of Mines has investigated the reliability of mine fire detection and suppression 
systems and the effectiveness of inspection and maintenance practices. Interviews with Mine Safety 
and Health Administration (MSHA) inspectors and field data indicate that reliability of mine fire 
suppression systems could be improved. Limitations of current inspection and maintenance prac- 
tices are discussed. Predictive diagnostics methods were developed and were tested in the field. 
The predictive diagnostics method employs functional parameter measurement to predict wear- 
out-related failures. Using these techniques, an intermittent system electrical relay failure was 
diagnosed and corrected. An impending actuator failure was also noted. Limited field data indicate 
a point estimate reliability of 75 pet for water deluge type systems. Reliability can be improved 
by careful adherence to standard maintenance and testing procedures, and by applying preventive 
maintenance techniques. 



INTRODUCTION 



Parts 57 and 70 of title 30 of the Code of Federal Regulations 
(CFR) requires the installation and maintenance of mine fire sup- 
pression systems in various locations in metal and nonmetal, and 
coal mines. The CFR testing and maintenance requirements 
reference various National fire codes, which in turn, reference 
manufacturers recommendations. Manufacturer testing and 
maintenance recommendations vary from product to product, and 
may not allow for system degradation due to the severe mine 
environment. Recent MSHA concern over the apparently low 
reliability of mine fire suppression systems, and maintenance in- 
consistencies, has led to the Bureau of Mines initiating a study of 
maintenance and testing for mine fire suppression systems. This 
paper presents a general summary of this research. 



The objective of this research program was to improve the 
reliability of underground fire suppression systems by developing 
systematic inspection and testing procedures and by developing 
predictive diagnostic techniques. Predictive diagnostics would allow 
correction of impending reliability failures before they occur. The 
combination of systematic testing procedures with predictive 
diagnostics should virtually eliminate system unreliability. The 
research program consisted of three parts: (1) system and failure 
mode identification, (2) system reliability testing design, and (3) 
in-mine evaluation of test concepts. 



REVIEW OF FIRE SUPPRESSION SYSTEM FUNCTION 



There are two basic types of fire suppression systems, the 
automatic sprinkler type and the fire sensor actuated types. The 
automatic sprinkler system type uses heat activated sprinkler heads, 
each opening individually in response to fire. These systems are 
designed to contain fires, and to minimize damage, but may not 
toally extinguish fires. Automatic sprinklers have the advantage of 



'General engineer. Twin Cities Research Center, Bureau of Mines, Minneapolis, 
MN. 



efficiently putting water where heat occurs, but may lag behind fires 
where rapid flame spread occurs. 

Fire sensor actuated systems use fire sensors that activate a 
separate fire suppression system. Typical suppression systems 
include water deluge systems (directed open water nozzles), high 
expansion foam generators, and dry chemical systems. Fire sensing 
devices include rate of rise heat detectors, bimetallic heat switches, 
and optical flame detectors. The suppression agent systems are ac- 
tivated by an actuation valve controlled by electrical or mechanical 



43 



circuits. These systems are designed to protect a defined area, and 
to prevent rapid fire spread by applying extinguishant ahead of the 
fire. Water deluge systems are the most common suppression 
systems of this type. Dry chemical systems are usually used in areas 
where water freezing may be a problem, such as surface belt drive 
locations. High expansion foam systems are usually used where 
water availability is limited, such as parts of the western United 
States, or as part of a multiple agent fire fighting plan. 

Because of the added complexity of the separate fire sensing 
and suppression systems, the reliability of sensor actuated type 
systems is generally lower than automatic sprinklers. Because of 
this complexity, the primary focus of this work was on these 



systems, although the general concepts can be applied to all fire 
suppression system types. 

Figure 1 shows a typical water deluge system designed for pro- 
tection of a conveyor drive. Notice the separate heat sensing and 
water valve actuation circuits. The heat sensors normally open and 
close at the actuation temperature of 180° F. The closing sensor 
circuit activates a latching relay that (1) closes an audible and visible 
alarm circuit, (2) activates the suppression agent discharge system 
through an electronic servovalve (motor or solenoid), and (3) deac- 
tivates the belt drive motor. The distribution lines have a manually 
operated valve to bypass the sensor based actuation circuit. 



Water deluge 
control box 




Electric 
water 
valve 



Heat 
sensors 



Figure 1.— Water deluge system. 



44 



SYSTEM AND FAILURE MODE IDENTIFICATION 



In order to assess system types and potential failure modes, 
MSHA electrical inspectors were interviewed as part of a 1986 elec- 
trical retraining session. The purpose of this interview was to identify 
sensor activated system types and common failure modes. The 
system types identified in the course of this interview were primarily 
water deluge (90 pet) with some dry chemical and high expansion 
foam systems. This information was used to focus the research on 
the most common system types. 

Table 1 summarizes the results of the MSHA interviews, 
describing common failure types for water deluge, dry chemical, 
and high expansion foam system types. Common problems included 
dead batteries, open contact points, and clogged nozzles. The general 
consensus was that fire suppression system reliability could be im- 
proved through better system maintenance. 

Preliminary field system data were collected in four mines in 
the Pikeville, KY, area in July 1986. This area was selected as a 
representative eastern U.S. underground coal mining area. Data 
collected included system types in place, system installation prac- 
tices, temperature and humidity conditions, and potential fire sup- 
pression failure modes. High humidity was observed at each mine. 
System access problems were noted, in particular, heat sensors and 
distribution nozzles were difficult to access because of proximity 
to the moving belt and the presence of guards. Water deluge nozzle 
caps were not observed on any of the systems. Electronic control 
wires and electronic control box installation was often improper. 
Lines were often strung either too tautly or unsecured, and control 
boxes were found loosely hanging from control lines. These obser- 
vations and subsequent discussions with safety personnel regarding 
reliability problems were consistent with the previous MSHA in- 
spectors survey. 

Contacts with several system manufacturers were made to deter- 
mine the current maintenance and inspection procedures used for 
various system types. Schematics were also obtained. Functional 



testing was the most commonly recommended system testing 
method. The manufacturers suggested procedures were a useful 
starting point, 2 but did not fully meet the test method design objec- 
tives for inspection procedures, particularly the aspect of predict- 
ing impending failures. 

Two commercially available representative water deluge 
systems were acquired to evaluate normal operating conditions and 
functional parameters. This laboratory evaluation was the basis for 
further test system development. The two systems had functional 
similarities with each other and with other systems in the field. Close 
examination of functional components indicated that the most prob- 
able failure types would be either caused by electrical discontinuity, 
mechanical seizure, or battery deterioration. Each of these failure 
types would be accentuated by the wet and dust-contaminated con- 
ditions in mines. 



Table 1 .—Survey of common failure fire suppression failure types 



System type 
Water deluge systems. 



Dry chemical systems . 



High expansion foam 
systems. 



Failure modes 

Relays inoperative. 

Dead batteries. 

Clogged water lines. 

Spray nozzles clogged. 

Moisture in control box. 

Battery corrosion. 

Sticky solenoid water valve. 

Insufficient or no water supply. 

Heat sensor failure. 

Broken wiring. 

Burned out belt shutdown switch. 

Trigger device seizure. 

Moisture and corrosion in control box. 

Blocked or broken distribution lines. 

Generally do not function correctly. 



SYSTEM RELIABILITY TEST DESIGN 



The survey and field observations were essential for the 
development of a practical inspection procedure that would have 
a high probability of identifying critical system failures and which 
would predict impending failures. Testing that predicts impending 
failures is important because preventative maintenance can then be 
used to correct problems prior to system failure. It is also desirable 
for the inspection procedures not interfere with conveyor belt pro- 
duction except when verifying shutdown relay operation. The 
approach should not be labor or cost intensive, or require destructive 
testing (i.e., dry chemical system discharge), and be generic, for 
applicability to various system types. 

Reliability is the probability that an item will perform its 
intended function for a specified period under stated use conditions. 
There are three general types of product failures: Infant mortality 
failures, random product life failures, and wear-out failures. These 
failure types are common for populations of product and are shown 
in the general bathtub curve in figure 2. The dependent variable 
is the average probability of failure for product populations as a 
function of time. This conventional perspective is useful for product 
populations but not for discrete product units. 



A given product unit under specified conditions will have only 
one discrete failure time. The failure will either be due to normal 
wear out or due to a discrete random occurrence. Normal wear out 
is caused such gradual factors as mechanical wear, electronic com- 
ponent corrosion, or general mechanical or thermal fatigue. Ran- 
dom failures occur all at once and are typified by mechanical 
breakage, electrical burnout, or some unforeseen environmental 
event. Wear-out-related failures can be predicted by quantifying 
the degree of wear. The stated objective of this work was to iden- 
tify possible precursors to system failures. 

There are three methods of determining product functionality: 
Subjective observation, discrete testing, and continuous functional 
variable measurement. These three concepts can be illustrated us- 
ing the simple example of carbon-zinc batteries. Functionality can 
be subjectively inferred from the appearance, i.e., presence of cor- 
rosion on the case would indicate bad batteries. Functionality can 
be tested by determining the ability of the batteries to light a bulb. 



Manufacturer recommendations should be followed in any inspection and 
maintenance program. The ultimate object of this work is to point out methods for 
manufacturers to improve recommended maintenance and inspection techniques. 



45 



-Useful life 




TIME 





/ |Time 

/ lOf 

Electric / [system 
water valve / failure 
voltage-x / 

Warning \^s — level 




I time 



Figure 2.— Failure probability over product population life. 



Figure 3.— Wear-out trending to predict failure time. 



This type of testing is called go, no-go testing or discrete testing. 
Most functional tests are discrete by nature (the system either works 
or it does not). Functionality can be determined by continuous 
variable measurement. This type of testing would involve the 
measurement of battery power, and would indicate a quantitative 
level of performance. 

Continuous variable measurement is useful to determine safety 
margins, as well as quantifying system wear-out trends. The analysis 
of system wear is also called functional (reliability) trending. 
Random failure events, as previously defined, cannot be predicted. 
Frequent inspections are therefore necessary to minimize the effects 
of these failures. 

Many sensor actuated fire suppression system failure modes 
are wear-out related, and therefore predictable. Battery internal 
resistance will often gradually increase, relay and sensor point con- 
tacts and wire connectors will gradually corrode, moving parts in 
relays and water valves will gradually become more sticky. These 
wear-out modes constitute a large percentage of fire suppression 
system failures. 

Figure 3 illustrates the concept of failure prediction. In the 
figure, battery voltage and electric water valve actuation voltage 
are trended over time. If the battery voltage drops below the solenoid 
actuation voltage, the system will not work. Battery voltage 3 will 
decrease over time because of wear out, while electric water valve 
actuation voltage will increase because of corrosion and water 
chemical deposition. By tracking these performance parameters it 
should be possible to predict the time to failure. 

Failure prediction is not possible for random failure events, 
such as control wires being cut by roof falls or machinery, or by 
rock dust application clogging deluge nozzles. Visual observation 
and functional testing are the best ways to assess these conditions. 
The inspection interval for random failures types should be at least 
weekly. 4 Specific items to check include battery and auxiliary power, 



electric continuity, and suppression agent distribution lines. 
Manufacturer's inspection recommendations outline visual and func- 
tional testing methods, such as the following. 

Water Deluge Inspection 
Weekly Test 

1 . Test batteries by pushing battery test switch. Batteries are good 
if light emitting diode battery indicator light comes on. 

2. Test heat sensor and solenoid valve circuit continuity by 
pushing circuit test switch. Circuits are good if sensor and solenoid 
circuit light emitting diode indicator lights turn on. 

3. Check circuit and water hose runs for looseness, breakage, 
or possible abrasion. 

4. Check to see that all nozzle blowoff dust covers are on. 

5. Open strainer flush out valve to check for good water flow 
and low sedimentation. 

Table 2 illustrates the various methods of reliability assessment, 
and lists advantages and disadvantages. MSHA inspection guidelines 

Table 2.— Comparison of reliability assessment techniques 



Assessment method Advantages 



Disadvantages 



'Actual measurements are of battery internal resistance, which correlates to decreased 
battery energy potential. 

'This is the current visual inspection interval for which written visual inspection 
records are required. 



Subjective observation 
problems (i.e., visual 
examination). 

Discrete testing (i.e., 
go, no-go testing or 
functional test). 



Continuous functional 
parameter measure- 
ment (i.e., measure- 
ment of battery 
power). 



Fast decisionmaking. 
Quickly find faults. 



Systems can be tested 
functionally. Accurate 
assessment of current 
condition. 



Important components 
can be individually 
tested. Wear-out 
trends can be 
assessed. Statistically 
efficient. 



Not precise. Easy to 
miss failures. 



Cannot isolate in- 
dividual bad parts. Im- 
pending failures not 
detectable. May be 
destructive to system 
tested. Chance that 
intermittent failures 
will be missed. 

Need to separate com- 
ponents. Time con- 
suming. Need to 
understand compo- 
nent interrelationships. 



46 



call for weekly visual (subjective) inspections, with annual func- 
tional (discrete go, no-go testing) of water deluge systems. Func- 
tional testing is useful, but will not guarantee future reliability. 
Visual observations are effective for obvious defects such as broken 
wires or cut distribution lines, but will not detect loose connectors 
or poor electrical continuity. A good maintenance program should 
include a combination of subjective observation, functional testing, 
and quantitative function measurement. 

In order to investigate the effectiveness of predictive diagnostics 
techniques for mine fire suppression systems, several test devices 
were designed and constructed. The purpose of these devices was 
to quantitatively measure those functional parameters that may show 
signs of wear out. The devices constructed included a portable 
battery powered oven for heat detector actuation temperature 
measurements; a variable power supply for measuring electric water 
valve actuation energy; a variable voltage supply for measuring relay 
actuation voltages; a constant current supply for determining con- 
tact point line resistance and control line continuity; and a 1-s pulsed 
load circuit for measuring battery internal resistance. Figure 4 shows 
the portable sensor oven; figure 5 shows the combination power- 
voltage-current control box and the battery tester. Readouts were 
obtained with intrinsically safe multimeters. 

The devices were all built using intrinsically safe design prin- 
ciples, although this was not required for the locations covered. 
These circuits were relatively simple in design and construction. 




Figure 4.— Portable sensor oven. 



m*#!&®mj#**?< 




Figure 5.— Power-voltage-current control box and battery tester. 



47 



IN-MINE EVALUATION OF TEST CONCEPTS 



In order to test the effectiveness and practicality of the test 
devices and techniques, data were collected from four mines in 
Colorado and Utah. A test procedure had been outlined in the 
laboratory, but it was necessary to obtain field reliability data, as 
well as to develop a practical field approach to system testing and 
maintenance. MSHA assisted in the selection of four representative 
western U.S. mines, to complement the data from the eastern U.S. 
mines. 

Several fire suppression system installation types were observed 
including automatic sprinklers, dry chemical systems, high expan- 
sion foam systems, and water deluge systems. Equipment protected 
included electrical transformers, motor control stations, rock dust 
application machines, and belt drives. The condition of the fire 
suppression systems observed were similar to those in the eastern 
U.S. mines. 

The procedure used to measure functional parameters was as 
follows: 

1. Disconnect and remove batteries. Measure battery internal 
resistance. 

2. Place heat sensor oven on accessible sensors, measure 
actuation temperatures. 

3. Energize relay with variable voltage device, measure actuation 
voltage, and contact point resistance. 

4. Measure water valve actuation power requirements using 
variable power source. 

5. Carefully reassemble system. Perform functional test to assure 
correct assembly. 

Additional observations made included the condition of the 
sensor and control wires, the condition of the suppression distribu- 
tion lines, the water flow rates, evidence of nozzle clogging, the 
presence of nozzle covers, general control box condition, and obser- 
vation of contamination or corrosion. 

Four similar systems in two mines were tested using the 
procedures outlined. The first test took 1 h; the final test took only 
10 min because of improved organization. Data for the parameters 
tested are presented in table 3. The systems generally showed little 
variability even after over 1 yr of field installation. 



Functional parameter measurement successfully diagnosed an 
intermittent relay closure failure apparently caused by rock dust 
contamination on the final system tested. The contact point resistance 
varied between 0. 1 and 100 ohms. The readings were so high in 
some cases that the test meter initially appeared to be broken. The 
functional test confirmed this condition, in that the system functioned 
on the first test, not on the second test, and about every other time 
for subsequent tests. Given these results the system would have 
passed the annual functional test, but would not have worked in 
the event of a real fire. This intermittent condition was pointed out 
to mine personnel for correction. 

Analysis of test 3 revealed an alarming near-failure condition. 
Test 3 was conducted on a dry chemical system. The battery voltage 
was 12.3 V under a no-load condition, with a measured dual battery 
internal resistance of 1.0 ohm. The battery voltage would be at 
1 1 .6 V under the 0.71 -A load of the dry chemical actuation plunger. 
Since the actuation plunger requires a minimum of 1 1 .4 V to fire, 
the system appeared very close to failure (intersecting lines in 
figure 3). 



Table 3.— Summary of field test data 

Parameter Test 1 Test 2 Test 3 1 Test 4 

Battery internal resistance: 
A: 

Resistance ohms.. 1.4 1.6 2.1 1.8 

Voltage V.. 13.3 12.3 12.3 12.3 

B: 

Resistance ohms.. 1.5 1.8 1.8 1.8 

Voltage V.. 13.0 12.3 12.3 12.4 

Heat sensor actuation temperature . °F . . 1 82 ( 258 263 273 

[ 260 273 266 

/ 263 272 273 

Relay closure voltage V. . NM NM 7.5 7.2 

Relay contact point resistance . .ohms. . NM NM 0.024 ( 4.5 

I 0.01 

| 100 

Water valve actuation power: ' 

Voltage V. . 8.23 7.39 2 11.4 7.8 

Strength A.. 0.61 0.53 *0.71 0.55 

Time to complete min . . 60 60 30 10 

NM Not measured. 1 Dry chemical. Actuation plunge. 



RESULTS AND CONCLUSIONS 



The results from the field work were mixed. It could be inferred 
that one in four systems is not reliable, but given the limited sample 
size (four) it is likely that the actual percentage of unreliable systems 
is somewhat greater or less than 25 pet. The correct diagnosis of 
the contact point failure and the near-failure condition was signifi- 
cant, since these confirmed the effectiveness of functional parameter 
measurement. 

The test procedures developed effectively supplement the func- 
tional testing method, but the procedures may be too complicated 
and time consuming to be followed. Because functional parameter 
measurement requires system disassembly, careful circuit 
reassembly is critical; assembly errors could potentially introduce 
more failures than averted through the techniques. It would be possi- 
ble to incorporate the test circuit concepts in the design of the fire 
suppression system control circuits, but this may increase system 
costs. Some manufacturers include some effective test circuits at 
this time; the battery test indicator as shown in figure 6 is one 
example. A suggested incorporation would be to use a series resistor 




Figure 6.— Typical commercial battery test circuit. 



48 



in the battery activated water valve circuit during functional testing. 
This resistor would provide a margin of safety in the event of a 
real system actuation (without test resistor). 

Predictive diagnostics will provide the most benefit when 
careful manufacturing, installation, and maintenance procedures are 






followed. Predictive diagnostics has the potential of being an 
effective failure prevention technique. The responsibility for system 
reliability is shared by manufacturers, end users, and regulatory 
agencies. A better understanding of mine equipment reliability 
failure modes will result in a safer mining industry. 






49 



DIESEL EXHAUST CONDITIONING SYSTEMS FOR FIRE 
AND EXPLOSION CONTROL IN GASSY MINES 



By Kenneth L. Bickel 1 



ABSTRACT 

Diesel-powered mining equipment operating in underground gassy mines must be equipped 
with control devices to lower surface and exhaust temperatures and prevent flames and sparks from 
being emitted to the mine atmosphere. The primary control device used to meet these requirements 
is the water scrubber. Water scrubbers have performed well over a number of years, but they do 
have disadvantages, which include frequent maintenance, large size, and high water consumption. 

The Bureau of Mines, as part of its program in diesel exhaust control technology, is conducting 
research in cooperation with mining companies, equipment manufacturers, and equipment sup- 
pliers on a promising alternative to water scrubbers; the dry exhaust conditioning system. The dry 
exhaust conditioning system will cool the exhaust and suppress flames and sparks without direct 
contact between the exhaust gas and water. 

This paper describes the dry exhaust conditioning system, and discusses an ongoing program 
to evaluate the system for use on a large engine in a gassy noncoal mine, and for a small engine 
operating in a coal mine. 



INTRODUCTION 



Diesel-powered mining equipment offers a number of ad- 
vantages over other types of materials handling equipment. The 
mobility, versatility, fuel economy, and long service life of diesel - 
powered equipment have allowed it to gain wide acceptance in 
underground nongassy mines. 

Methane and combustible dusts present in gassy mines pose 
fire and explosion hazards that must be considered in the design 
of diesel-powered equipment. Hot engine and exhaust system sur- 
faces, as well as hot exhaust gases, must be cooled to prevent fires 
and explosions. Provisions must be made to prevent the discharge 
of flame or sparks to the mine atmosphere. 

The use of diesel equipment in underground mines is governed 
by regulations in the U.S. Code of Federal Regulations (CFR), title 
30. Part 36 outlines the requirements diesel equipment must meet 
to be approved and certified as permissible for use in gassy non- 
coal mines. Part 36 considers toxic or objectionable gases, the 
ignition of flammable gas mixtures by the engine or electrical equip- 
ment, fire hazards presented by combustible materials coming in 
contact with the equipment, and mechanical hazards (I). 2 In October 
1987, new standards for classifying gassy noncoal mines were 
enacted (2). A chapter specifically for diesel equipment in 
underground coal mines has not yet been established, but equip- 
ment to be used in gassy areas of underground coal mines is cur- 
rently being tested by the Mine Safety and Health Administration 
(MSHA) in accordance with part 36, except the maximum allowable 
surface temperature is reduced from 400° to 302° F. 



'Mining engineer, Twin Cities Research Center, Bureau of Mines, Minneapolis, 
MN. 

2 Italic numbers in parentheses refer to items in the list of references at the end of 
this paper. 



Part 36, subpart B, gives construction and design requirements 
for diesel-powered equipment in gassy noncoal mines. Section 36.25 
outlines requirements for the exhaust system. These include an 
exhaust flame arrestor, surface temperature requirements, tight 
joints, exhaust gas dilution, the ability of the exhaust system to with- 
stand an explosion, and an exhaust cooling system. Cooling must 
be obtained by passing the exhaust through a conditioner that con- 
tains water or a dilute aqueous chemical solution, or a spray of water 
or aqueous solution. These conditioners, referred to as water scrub- 
bers, are used in those areas of gassy noncoal mines and coal mines 
where permissible equipment is required. 

All equipment presently certified under part 36 use diesel 
exhaust gas water scrubbers. While water scrubbers have proven 
to be effective in cooling exhaust and acting as flame traps, they 
do have a number of problems that have been described in a Bureau 
report (3). These problems include sludge and mineral deposit 
buildup on internal baffles and passages, premature failure at mount- 
ing points and welds, severe corrosion of mild steel welds and com- 
ponents, and pitting corrosion of stainless steel components. 

The use of water scrubbers has other disadvantages. Scrub- 
bers consume large amounts of water, and scrubber solution must 
be added frequently. Entrained water in the exhaust can condense 
when discharged to the atmosphere, obstructing visibility in the 
mine. Vehicle design and operator field of vision may be affected 
by the large size of scrubbers, and the back pressure induced on 
the engine by the scrubber may affect its performance. 

The amount of water consumed in a water scrubber is directly 
proportional to the horsepower rating of the engine. Water scrub- 
bers are designed to operate for one shift before more scrubber solu- 
tion must be added. Engines used in underground coal production 



50 



equipment generally do not exceed 150 hp, but the frequent addi- 
tion of scrubber solution, flushing of accumulated sludge, and repair 
and replacement of corroded parts is costly. Several coal mine 
operators have estimated scrubber maintenance costs to range from 
$500 to $1,500 per month for each scrubber (4). 

Engines in some gassy mines, such as domal salt mines, may 
have ratings in the 600- to 750-hp range. Engines used in oil shale 
mines are of comparable size. Oil shale mines are currently not 
declared gassy, but they may be declared gassy sometime in the 
future if methane is found as mining progresses deeper into the oil 
shale formation. Water scrubbers required for the engines used in 
these mines would be very large, and may obstruct operator field 
of vision. The water consumption would also be very high. It has 
been estimated that in an oil shale mine using 40 vehicles with 
engines in the 750-hp range, the annual water consumption would 



be in excess of 25 million gal, which is equivalent to 77 acre-ft 
of water (5). 

The need for an alternative to water scrubbers for large engines 
was recognized, and the Bureau initiated a project to find an alter- 
native method of cooling diesel exhaust (5). The most promising 
alternative identified was the dry exhaust conditioning system. The 
dry exhaust conditioning system was chosen because it does not 
consume water, does not require extensive development, and prom- 
ises to require less maintenance than a water scrubber. 

The following discussion describes two different dry exhaust 
conditioning systems for underground gassy mine applications. One 
system was designed for use with large engines for gassy metal or 
nonmetal mine application, such as oil shale or salt mines. The other 
system is for small engines used in coal mines. 



DRY EXHAUST CONDITIONING SYSTEM 



Any explosion-proofing system must have the capability to con- 
trol surface and exhaust temperatures, prevent sparks and flame 
from being emitted to the mine atmosphere, and must maintain struc- 
tural integrity in the event of an internal explosion. After identifying 
and evaluating six different concepts for cooling exhaust gas, the 
dry exhaust conditioning system was chosen as the most promising 
concept for further development (5). 

The dry exhaust conditioning system does not require direct 
contact between the exhaust gas and water. Dry system technology 



was originally developed for use in other industries where com- 
bustible gases or dusts may be present, such as munitions factories 
or offshore drilling platforms. These systems have recently been 
adopted for use in European underground coal mines, but have not 
been approved for use in U.S. mines. Two companies presently 
manufacture dry exhaust conditioning systems that may be adapted 
for use in the United States (6-7). 

Figure 1 illustrates a dry exhaust conditioning system with a 
certified diesel engine. The exhaust from the engine may pass 



Certified 
diesel 
engine 



Heat exchanger 




Cool exhaust 



Flame trap 



Figure 1.— Schematic of dry exhaust conditioning system. 



51 



through a water-cooled manifold and piping to a heat exchanger, 
or the exhaust manifold and heat exchanger can be incorporated 
into one component. One or more heat exchangers cool the exhaust 
gas. The heat from the exhaust gas is dissipated using either the 
engine radiator and coolant, or a completely separate cooling system 
using a second radiator and additional coolant. The water jacketing 
of exhaust components and waste heat from the heat exchanger add 
a significant heat load. If the engine's cooling system is used, it 
will need to be larger than standard by 70 pet or more. Flame 
arrestors downstream of the heat exchangers prevent any flames 
from being emitted to the mine atmosphere. 

The dry exhaust conditioning system has the potential for 
needing much less maintenance than water scrubbers. It consumes 
no water, does not require water level floats and control valves, 
and its components are smaller than a water scrubber, allowing more 
flexibility in placement of the system on the vehicle. 



DRY EXHAUST CONDITIONING SYSTEM FOR 
LARGE ENGINES IN GASSY NONCOAL MINES 

In 1986, the Bureau entered into a cooperative research program 
with the Colorado Mining Association, the Caterpillar Equipment 
Co. , Wagner Equipment Co. , Union Oil Co. , and MSHA to design, 
fabricate, and laboratory and in-mine test a dry exhaust condition- 
ing system for a 50-st-capacity haul truck used in an underground 
oil shale mine. The 31 -month project is being performed under 
contract to J.F.T. Agapito and Associates, and is scheduled to be 
completed in 1988 (8). 



The system was designed for a Caterpillar 3412 turbocharged 
engine rated at 650 hp installed in a Caterpillar 773B haul truck. 
It was designed to meet the 400° F surface temperature require- 
ment, and to cool the exhaust gas to the same temperature. 

After passing through the engine's exhaust manifold and turbo- 
charger, which are water jacketed, the exhaust enters another 
manifold where it is directed to three shell-and-tube heat exchangers. 
Three heat exchangers are required to cool the high exhaust flow 
at rated speed and load conditions. The exhaust gas passes through 
the tubes of the heat exchangers, with the cooling liquid circulating 
through the shell. The coolant for the entire dry system is circulated 
through a radiator, and is kept completely separate from the engine 
cooling circuit. The cooled exhaust leaves the heat exchangers and 
passes through a device that functions as both a flame and spark 
arrestor. Finally, the exhaust is dumped in front of the dry system 
radiator, where it is blown upwards and away from the truck. 

The system has completed a laboratory evaluation at MSHA's 
Approval and Certification Center, and has been installed on a haul 
truck at Union Oil's Long Ridge oil shale mine. The laboratory 
evaluation included a series of explosion tests, a surface temperature 
test, and an endurance test. The system performed well during the 
laboratory test, although a soot buildup problem in the heat 
exchanger tubes resulted in the exhaust gas exceeding 400° F, 
indicating that more heat rejection capacity may be required. 

Figure 2 shows the Caterpillar 3412 engine and dry system on 
the laboratory test stand. Currendy, the system is undergoing testing 
in the mine. After completion of the test, the system will be 
removed, and a complete evaluation of each component will be made 
to determine any damage that may have occurred during the in- 
mine evaluation. 




Figure 2.— Large dry exhaust conditioning system and Caterpillar 3412 engine undergoing laboratory testing at MSHA's Approval 
and Certification Center. 



52 



DRY EXHAUST CONDITIONING SYSTEM FOR 
SMALL ENGINES IN COAL MINES 

In 1987, a cooperative project to test a dry exhaust condition- 
ing system for use on small engines (up to 150 hp) in coal mines 
was initiated. The project was established with the assistance and 
cooperation of manufacturers of diesel engines, mining machines, 
exhaust control devices, and coal mine operators. The Colorado 
Mining Association represents these participants, and coordinates 
activities among them, the Bureau, and MSHA. The objectives of 
the program are to laboratory and in-mine test a dry system with 
and without a diesel particle filter (DPF) installed downstream of 
the dry system. The DPF is a device designed to remove approx- 
imately 90 pet of the diesel particulate matter (DPM) from the 
exhaust before its discharged into the mine air (9). 

The dry exhaust system was designed for a Caterpillar 3306 
engine installed in a Jeffrey 4114 ramcar. The engine's exhaust 
manifold is replaced by a fin-and-tube heat exchanger, where 
exhaust gas cooling takes place. The engine coolant passes through 
the water-jacketed components and the heat exchanger. Cooling the 
exhaust and engine and exhaust system surfaces to below 302 ° F 
requires an additional 70 pet in heat rejection capacity over the 
standard engine cooling system. A spaced-plate flame arrestor is 
located downstream of the heat exchanger. 



Diesel engines emit large amounts of DPM. Tailpipe emissions 
may exceed 400 mg/m 3 . DPM has been shown to make up 40 to 
80 pet of the respirable dust found in coal mines using diesels, and 
therefore is indirectly regulated under the 2-mg/m 3 respirable dust 
coal mine standard. There is also a potential health risk associated 
with exposure to DPM (4). For these reasons, the dry system is 
being tested with a DPF to determine the feasibility of having an 
exhaust system that not only controls fires and explosions, but also 
removes DPM. 

The DPF consists of a cellular ceramic substrate in a stainless 
steel housing. It is sized so that it will operate one to two shifts 
before it requires regeneration. Regeneration is the cleaning proc- 
ess where the DPM collected in the substrate ignites and burns, 
leaving very little ash. Regeneration can occur on a vehicle, if the 
vehicle has a heavy duty cycle where its exhaust temperature exceeds 
950° F for sustained periods of time. However, for this applica- 
tion where the exhaust is cooled, regeneration must be done off 
of the vehicle; that is, the DPF must be removed from the vehicle 
for regeneration. 

The research program is divided into two parts consisting of 
a laboratory test and in-mine evaluation. The laboratory test is being 
conducted by both the Bureau and MSHA. The Bureau is currendy 
testing the system, with and without the DPF. Figure 3 shows the 
dry system and DPF installed in the Bureau's engine testing facility. 






I ' ~S , 



fwfj 




Figure 3.— Small dry exhaust conditioning system and Caterpillar 3304 engine undergoing Bureau laboratory testing. 



53 



During the Bureau's evaluation, surface and exhaust temperatures, 
and exhaust pressure buildup across components are being 
measured. The major emphasis is on determining the efficiency of 
the DPF in removing particulate, and on characterizing the effect 
of the dry system and DPF on the size distribution and chemical 
composition of the diesel particulate. MSHA's test program will 
evaluate the dry system to determine if it will meet the requirements 
of CFR part 36. It will consist of a series of tests that include sur- 



face temperature, cooling efficiency, endurance, induced faults, 
safety shutdown, and spark arrestor tests (4). 

After satisfactory completion of the laboratory evaluation, the 
dry system and DPF will be installed on a Jeffrey 41 14 ramcar in 
a coal mine in the western United States, for a period of 6 months. 
The system and DPF will be evaluated for its durability and per- 
formance under operating mine conditions (4). 



SUMMARY 



Water scrubbers are presently being used to cool exhaust and 
act as flame and spark arrestors on diesel-powered equipment used 
in gassy mines. The problems associated with their use led the 
Bureau to initiate a program to develop an alternative to water scrub- 
bers that could be used in coal mines and gassy noncoal mines. 
The dry exhaust conditioning system was selected as a promising 
alternative because it offers the potential for explosion-proofing a 
diesel' s exhaust system without consuming water, and with much 
less maintenance than that required of a water scrubber. Two 



cooperative research projects are under way to evaluate dry systems 
for different applications. One project is evaluating the dry exhaust 
system for use with a 650-hp engine on a haul truck operating in 
an underground oil shale mine. The other is evaluating the system 
for use on a 150-hp engine on a haulage vehicle operating in a coal 
mine. It is being evaluated with a DPF to determine if an exhaust 
control system can be developed that will control fires and explo- 
sions, and remove diesel particulate matter before it is emitted into 
the mine air. 



REFERENCES 



1. U.S. Code of Federal Regulations. Title 30— Mineral Resources; 
Chapter 1— Mine Safety and Health Administration, Department of Labor; 
Subchapter E— Mechanical Equipment for Mines; Part 36— Mobile Diesel- 
Powered Transportation Equipment for Gassy Noncoal Mines and Tunnels; 
July 1, 1984. 

2. Federal Register. U.S. Mine Safety and Health Administration (Dep. 
Labor). Safety Standards for Methane in Metal and Nonmetal Mines: Final 
Rule. V. 52, No. 126, July 1, 1987, pp. 24942-24951. 

3. Waytulonis, R.W., S.D. Smith, and L.C. Mejia. Failure Analysis of 
Diesel Exhaust-Gas Water Scrubbers. BuMines RI 8682, 1982, 19 pp. 

4. Waytulonis, R.W., and G.J. Dvorznak. New Control Technology for 
Diesel Engines Used in Underground Coal Mines. Paper in Proceedings 
of the 3d Mine Ventilation Symposium (Penn State Univ., University Park, 
PA). Soc. Min. Eng. AIME, 1987, pp. 279-285. 

5. Paas, N. Explosion-Proofing of Large Vehicles (contract JO 113070, 
Foster-Miller, Inc.). BuMines OFR 205-84, 1984, 275 pp.; NTIS PB 
85-145803. 



6. Minecraft, Inc. Product brochure, 1986; available on request from 
K.L. Bickel, BuMines, Minneapolis, MN. 

7. Pyroban Corp. Product brochure, 1986; available on request from K.L. 
Bickel, BuMines, Minneapolis, MN. 

8. J.F.T. Agapito & Associates. Development of a Dry Exhaust Condi- 
tioner for Large Diesel Engines. Ongoing BuMines contract HO267001; 
for inf. contact K.L. Bickel, TPO, BuMines, Minneapolis, MN. 

9. Baumgard, K.J., and K.L. Bickel. Development and Effectiveness 
of Ceramic Diesel Particle Filters. Paper in Diesels in Underground Mines. 
Proceedings: Bureau of Mines Technology Transfer Seminar, Louisville, 
KY, April 21, 1987, and Denver, CO, April 23, 1987. BuMines IC 9141, 
1987, pp. 94-102. 



54 



SPONTANEOUS COMBUSTION SUSCEPTIBILITY 
OF SULFIDE MINERALS 



By G. W. Reimers 1 and W. H. Pomroy 2 



ABSTRACT 

Under certain conditions present in the underground mine environment, sulfide minerals can 
self-heat as a result of exothermic oxidation reactions. This self-heating can lead to spontaneous 
combustion of carbonaceous materials such as mine timbers or even the sulfide minerals themselves. 
This Bureau of Mines paper describes thermal gravimetric analysis (TGA) tests conducted to measure 
the reactivity of 10 sulfide minerals in terms of their ignition point. TGA results also indicated 
evidence of exothermic reaction at temperatures below the sulfide ignition point. An isothermal 
oxidation procedure was also used to obtain additional quantitative information on the reactive 
behavior of six pyrite samples during low-temperature oxidation to ferrous sulfate. 

Samples of pyrite and marcasite were found to be the most reactive sulfides and ignition point 
values below 300° C were measured by TGA. The two other iron sulfides, pyrrhotite and 
arsenopyrite, and the copper sulfides, chalcopyrite and chalcocite, were less reactive. Lead sulfide 
(galena) and the zinc sulfides (sphalerite and wurtzite) failed to ignite below 500° C. Molybdenite 
had an ignition point of about 375 ° C but failed to indicate any self-heating tendencies. Isothermal 
oxidation tests conducted on pyrite samples confirmed that this sulfide was reactive in moist at- 
mospheres. Conversion of up to 30 pet of the sulfide to sulfate was measured after 24 h at 100° C. 



INTRODUCTION 



When the proper conditions are present in the mine environ- 
ment, sulfide minerals can be susceptible to self-heating behavior 
that leads to spontaneous combustion and ultimately a mine fire. 
The Bureau of Mines conducted laboratory experiments to measure 
the reactivity of 10 sulfide minerals to identify the sulfides that would 
tend to pose the greatest hazard. These tests are part of a broader 
research effort to develop methods of controlling the oxidation of 
sulfides in underground mines and thereby reduce the possibility 
of spontaneous combustion mine fires. 

Preceding the current research effort, Ninteman (7) 3 conducted 
an extensive literature review of the nature and problems associated 
with the oxidation and combustion of sulfide minerals in 
underground metal mines. Sulfide oxidation is an exothermic proc- 
ess that produces heat, which, if not dissipated, causes a temperature 
rise within the mineral mass. If the temperature begins to rise, a 
thermal runaway situation may develop that will cause the sulfide 
and other combustibles to burn. An example of this thermal runaway 
situation was found in the Sullivan Mine (2-3) where, following 
the blasting of large blocks of reactive sulfide ore, self-heating was 
observed. If the mining activity was not of a sufficient rate to 
dissipate the heat, combustion of the ore resulted and fire-fighting 
measures were required. 

The prolonged incipient stage (weeks or months) and seem- 
ingly spontaneous appearance of these fires make their occurrence 



'Physical scientist. 

'Group supervisor. 
Twin Cities Research Center, Bureau of Mines. Minneapolis. MN. 

'Italic numbers in parentheses refer to items in the list of references at the end of 
this paper. 



difficult to predict and detect. Furthermore, these fires often start 
in abandoned, backfilled, and/or caved areas where abundant fuel 
is present but where access for fire fighting is difficult or in some 
cases impossible. Since detection and suppression are so difficult, 
a high priority is placed on fire prevention. However, the reac- 
tions and mechanisms giving rise to spontaneous combustion in 
sulfide ores are not well understood. Thus, current efforts by mine 
personnel to control spontaneous combustion problems, regardless 
of how well intended, are not always based on sound engineering 
and technical principles. 

One facet of the Bureau research involves monitoring the 
behavior of various sulfides exposed to oxidizing condition while 
being heated to their ignition point. This will provide a background 
of data to aid in the selection of fire prevention efforts having the 
maximum effectiveness. In this paper, the susceptibility of the 
various sulfide samples to spontaneous combustion was assessed 
by determining their respective ignition points. Using this criteria, 
samples with the lowest ignition points would be considered the 
most reactive. 

Data are excerpted from a Bureau study (4) on the analysis 
of the oxidation of pyrrhotite, marcasite, arsenopyrite, chalcopyrite, 
chalcocite, and galena. Additional data are included on the igni- 
tion points of pyrite, sphalerite, and molybdenite as determined by 
TGA. TGA analyses also suggest that sulfate formation could have 
a role in sulfide self-heating and the results of a quantitative method 
for following the oxidation of pyrite by measuring the conversion 
of insoluble iron sulfide to soluble iron sulfate are summarized. 
In this procedure, reactivity was gauged by the extent of oxidation 
during the defined test interval. 



55 



EQUIPMENT AND TEST PROCEDURES 



Ignition points were obtained using thermal analysis equipment 
consisting of the following Perkins Elmer 4 components: a TGS-2 
thermogravimetric analyzer in conjunction with a system 7/4 ther- 
mal analysis controller. Data from the analyzer were continuously 
fed to a thermal analysis data station (TADS), and the test results 
were displayed on paper by a TADS-1 plotter. 

In the TGA tests, a flow rate of 500 cmVmin was used for 
purging the system with nitrogen and for supplying air during the 
oxidation tests. To add moisture to the system, the oxidizing gas 
was bubbled into water contained in a heated flask and routed to 
the TGA furnace. The sample, approximately 40 mg in size, was 
loaded into the balance pan, and the system was closed by raising 
the furnace assembly into position. The assembly was then purged 
with nitrogen while the sample was heated to 100 ° C. At this point, 
the nitrogen was replaced with the oxidizing gas, and the sample 
was heated at 25° C/min while sample weight was monitored. 

Isothermal oxidation experiments were run with Lindburg 
model 55035, horizontal-hinged, tube furnaces having a bore of 
2.5 cm and a length of 33 cm, fitted with auxiliary temperature 
controllers. A single sample of 5 g was loaded into a 97- by 16- 



by 10-mm porcelain boat that was positioned at the center of the 
furnace inside a 2.2-cm-ID Pyrex glass tube. Nitrogen was used 
to purge the furnace tube while the sample was heated to the test 
temperature. The test atmosphere was then introduced at a flow 
rate of 500 cm 3 /min and timing of the test was initiated. For the 
tests run with air containing 5 pet water vapor, air was bubbled 
through water contained in a heated flask. When air containing 60 
pet water vapor was used, water was metered into a furnace 
assembly ahead of the test furnace that vaporized the water so that 
it could be carried along with the airflow to the oxidizing furnace. 
At the completion of each test, nitrogen flow resumed and the sample 
was moved to the cool end of the furnace tube. When cool, the 
sample was weighed and transferred to a bottle and sealed. 

The extent of pyrite oxidation for each test was determined by 
leaching with a known volume of water the soluble iron sulfate that 
is formed at that temperature. Solutions were collected by filtra- 
tion and then analyzed for their iron content. Knowing the iron con- 
tent of the starting sample and the amount of soluble iron formed 
during an oxidation test allowed calculation of the degree of ox- 
idation of the sulfide to the sulfate. 



SAMPLE PREPARATION 



The sulfides studied in this research effort were obtained from 
various sources including chemical suppliers, mineral specimen sup- 
pliers, and from a commercial mill. When necessary the sulfides 
were crushed in stages to minus 6 mm and the visible gangue was 
sorted out. Further size reduction was done in a Bleuler mill by 
milling 20-g batches for set time intervals ranging from 15 to 120 
min. To decrease the possibility of oxidation during this size- 
reduction step, the samples were milled and stored under heptane. 
Just prior to testing, a portion of the sample slurry was dried in 



air at room temperature, and then loaded into the test apparatus. 
Size analysis measurements were conducted on several of the 
sulfides, and the average particle size of these samples was in the 
range of 5.8 to 8.7 /urn and 3.4 to 4.8 ^m for the 15- and 120-min 
grinding times, respectively. Tables 1 and 2 list the partial chemical 
analysis of the various samples. X-ray diffraction (XRD) analysis 
was also conducted on the samples and the major constituents were 
verified. 



RESULTS OF THERMAL GRAVIMETRIC ANALYSES 



PYRITE 

Pyrite (FeS 2 ) samples collected from several sources were ex- 
amined by TGA in atmospheres of dry air or air containing water 
vapor. Figure 1 illustrates the results of tests conducted on pyrite 
sample A that was milled for 15 min. The Y-axis of the figure in- 
dicates the percentage of the original sample weight, measured over 
the temperature range of the test. The ignition point of pyrite was 
taken as the point on the curve where rapid weight loss occurred. 
Examination of the sample after TGA confirmed the conversion 
of the iron sulfide to ferric oxide (Fe 2 3 ) and was used as evidence 
to confirm ignition. 

Curve A indicates the results when the sample was oxidized 
with dry air, and under this condition an ignition point of 335° C 
was measured. In moist air (curve B) the ignition point increased 
slightly to 345° C. Curve B also indicates that the sample began 
to gain weight at about 290° C. This weight gain prior to ignition 
was observed for all the pyrite samples when they were oxidized 
in the presence of water vapor. Chemical and XRD analysis of pyrite 
samples that were oxidized in moist air for several hours at 
temperatures lower than the ignition point confirmed that the sulfide 
was partially converted to sulfate and its formation was responsi- 
ble for the weight gain. Increasing the milling time produces finer 
sulfide particles and the increased surface area would be expected 
to also increase the reactivity of the sulfide. 



Table 1. — Partial chemical analysis of iron sulfide samples, 
weight percent 



Eleme 


Jnt Pyrite 






Pyrrhotite 


Marca- 
site 


Arseno- 


A B C D 


E 


F 


A B 


pyrite 


Fe .. 


. . 43.1 38.6 40.2 45.4 


45.8 


45.8 


45.6 44.0 


45.6 


28.5 


S . . . 


. . 46.9 45.2 44.6 51.5 


52.2 


52.5 


30.2 24.2 


50.7 


13.4 


Si. . . 


2.2 1.7 3.8 .21 


1.2 


.85 


.77 8.5 


ND 


9.7 


Ca . . 


.61 1.5 .79 <.3 


<.3 


<.3 


1.8 1.9 


ND 


<.3 


Al. . . 


.54 .26 .46 <.2 


1.0 


.22 


<.2 2.4 


ND 


.31 


Zn 


.47 3.2 <.1 1.2 


<.1 


.14 


ND ND 


ND 


ND 


Pb .. 


.3 <.1 <.1 1.1 


<.1 


.22 


ND ND 


ND 


ND 


Cu . . 


.04 .1 .34 .02 


.1 


<04 


.1 .14 


ND 


ND 


Ni . . 


ND ND ND ND 


ND 


ND 


.0 3.8 


ND 


ND 


As .. 


ND ND ND ND 


ND 


ND 


ND ND 


ND 


27.6 


ND 


Not determined. 













Table 2.— Partial chemical analysis of copper, lead, zinc, and 
molybdenum sulfides, weight percent 



Elemt 


mt Chalcopyrite 


Chalcocite 
A B 


Galena 


Sphal- 
erite 


Wurt- 
zite 


Molyb- 


A 


B 


C 


denite 


Fe . . 


. . 37.4 


33.3 


30.8 


5.3 


5.6 


0.26 


1.1 


<0.02 


0.19 


Cu . . 


.. 21.2 


30.7 


33.3 


71.4 


68.0 


.24 


ND 


ND 


.035 


Pb . . 


ND 


ND 


ND 


ND 


ND 


82.2 


ND 


ND 


ND 


S . . . 


. . 31.8 


33.0 


31.6 


20.7 


19.0 


12.1 


32.2 


32.4 


37.7 


Al... 


.68 


<.20 


.31 


.37 


<.2 


ND 


ND 


ND 


ND 


Ca .. 


<.5 


<.5 


<5 


<.5 


<.5 


ND 


ND 


ND 


ND 


Si... 


1.7 


.48 


1.1 


.34 


2.2 


<.1 


.51 


ND 


.42 


Zn . . 


ND 


ND 


ND 


ND 


ND 


<.01 


60.8 


64.4 


ND 


Mo. . 


ND 


ND 


ND 


ND 


ND 


ND 


ND 


ND 


54.0 



■■Reference to specific products does not imply endorsement by the Bureau of Mines. 



ND Not determined. 



56 



Figure 2 shows the results of TG A tests condueted on pyrite 
sample E. milled for 15, 30, 60. and 120 min in the temperature 
range of 100 to 400"' C in dry air. The ignition points for the 
respective samples were 330°, 325°, 315°. and 270° C. These 
results illustrate the effect increasing the milling time (decreased 
particle size) had on lowering the ignition point. TGA tests of the 
remaining pyrite samples produced curves similar to those in figure 
1. however, there was a measurable difference in ignition points. 
Tabic 3 lists the ignition point values measured for milled pyrite 
samples in various oxidizing atmospheres. 

PYRRHOTITE 

Pyrrhotite (Fe„.,S„, with n ranging from about 5 to 16) was 
the second iron sulfide examined by TGA. Results of tests con- 
ducted on pyrrhotite A, milled for 15 min and oxidized in air and 
air containing 40 pet water vapor, are shown in figure 1A and in- 
dicate ignition point values of 415° C. For this pyrrhotite sample, 
the presence of water vapor failed to have a significant effect on 
the ignition point of the sample or the sample weight change at 
temperatures below 300° C. Increasing the milling time to 120 min 



Table 3.— Ignition point values of milled iron sulfides 
determined in selected oxidizing atmospheres 



„ , Milling 

Sample ,. a . 

r time, mm 

Pyrite: 

A 15 

B 15 

C 15 

D 15 

E 15 

30 

60 

120 

F 15 

Pyrrhotite: 

A 15 

120 
B 120 

Marcasite 15 

120 

Arsenopyrite 15 

120 

ND Not determined. 





Ignition 


poi 


nt, °C 




Dry 




Air 


with— 




air 


5 pet water 


40 pet water 


335 


345 






ND 


340 


380 






ND 


375 


380 






ND 


325 


340 






ND 


330 


350 






ND 


325 


ND 






ND 


315 


ND 






ND 


270 


ND 






310 


370 


345 






ND 


415 


410 






415 


365 


365 






360 


420 


410 






410 


ND 


ND 






380 


320 


ND 






255 


ND 


ND 






445 


390 


340 






330 



h r- 



~i r 



KEY 

A Air 

B Air containing 5 pet woter 




a- I05 



100 150 



200 250 300 

TEMPERATURE, °C 



Figure 1.— Thermal gravimetric analysis of pyrite sample A 
milled for 15 min. 



I- 

o 

I 

CC 

rr 

> 
a. 



IOO 



95 



no 



IOO 



£ 90 

< 
o 

1 80 



70 



1 1 1 

A Pyrrhotite A 


1 1 


1 1 
A 


- 


1 1 1 


B 


\ 



— I 1 — 

B Marcasite 



-i 1 1 r 




Pyrite E 




" 




D 


C 


KEY 






A 15 min 






8 30 min 






C 60 min 






D 120 min 


i 


i i 



250 

TEMPERATURE, 



Figure 2.— Thermal gravimetric analysis illustrating effect of 
milling time on oxidation of pyrite sample E in air. 



105 



- IOO 



95 



90 



85 



80 



— i 1 r 

C Arsenopyrite 



KEY 

A Air 

B Air containing 40 pet 
water 




IOO 150 200 250 300 350 400 450 500 
TEMPERATURE, °C 

Figure 3.— Thermal gravimetric analysis of (A) pyrrhotite sam- 
ple A milled for 15 min, (B) marcasite milled for 120 min, and 
(C) arsenopyrite milled for 120 min. 



57 



lowered the ignition point by about 50° C (table 3). Again, the ad- 
dition of water vapor had little effect on the ignition point, and no 
effect was observed on oxidation in the lower temperature regions. 
Results of TGA tests on pyrrhotite B milled for 120 min yielded 
an ignition point in dry air of 420° C, indicating less reactivity than 
the first sample. While water vapor had little effect on the ignition 
point of this sample it did, however, promote the weight gain of 
this sample at lower temperatures. XRD analysis of this sample 
after isothermal oxidation in moist air confirmed that ferrous sulfate 
was formed. Table 3 lists the ignition points obtained for the pyr- 
rohotite samples under various test conditions. 

MARCASITE 

Marcasite has the same chemical formula as pyrite but has a 
different crystal structure. A TGA test conducted on the marcasite 
sample indicated that it was the most reactive in terms of the lowest 
ignition point of the sulfides tested in this report. Figure 3fi illustrates 
the TGA results for the sample milled for 120 min and oxidized 
in dry air and air containing 40 pet water vapor. For this sulfide 
the presence of water vapor had a strong influence on the ignition 
point and was found to lower the ignition point to 255° C, which 
was 65 ° C lower than the value obtained in dry air. XRD analysis 
of the samples after testing indicated conversion of the sulfide to 
ferric oxide. Marcasite ignition point values determined for various 
conditions are listed in table 3. 

ARSENOPYRITE 

The final iron sulfide examined in this series was arsenopyrite. 
Arsenic replaces a sulfur in pyrite, making this mineral a 
sulfarsenide of iron (FeAsS). TGA results for a sample milled for 
120 min are illustrated in figure 3C. For this mineral 40 pet water 
vapor had the effect of lowering the ignition point from 390° to 
330° C and also caused the sample to gain weight at lower 
temperatures. Figure 3 C also illustrates an unusual double weight 
loss when water vapor was present. It was assumed that iron sulfide 
ignited first and was followed by the ignition of iron arsenide. XRD 
analysis of a sample oxidized to the completion of the first step 
indicated the presence of ferric oxide, but it failed to confirm if 
the sulfide or arsenide component of the mineral had oxidized. Table 
3 lists the ignition points measured for the arsenopyrite sample. 



Table 4.— Ignition point values of milled copper and 

molybdenum sulfides determined in 

selected oxidizing atmospheres 



Sample 



Milling 
time, min 



Ignition point, °C 



Dry 
air 



Air with— 



5 pet water 40 pet water 



Chalcopyrite: 

A 15 

30 

60 

120 

B 15 

120 

C 15 

120 

Chalcocite: 

A 15 

120 
B 15 

Molybdenite 15 

120 



365 
345 
335 
330 
385 
305 
365 
300 



(360) 
(335) 
(370) 

375 
375 



ND 
ND 
ND 
ND 
ND 
ND 
ND 
ND 



ND 
ND 
ND 

ND 
375 



360 
ND 
ND 
335 
385 
300 
355 
320 



(360) 
(365) 
(420) 

ND 
370 



ND Not determined. 

NOTE. — Values in parentheses represent conversion to sulfate. 









D 


C 




IUU 


KEY 










A I5n 


in 








9b 


B 30 n 


,in 










C 60 n 


lln 






8 




CI20n 
i 


1 1 









200 250 

TEMPERATURE, 



Figure 4.— Thermal gravimetric analysis illustrating effect of 
milling time on oxidation of chalcopyrite sample A in air. 



CHALCOPYRITE 

Chalcopyrite, an important ore of copper, is a sulfide of cop- 
per and iron and has a typical formula of CuFeS 2 . Results of TGA 
tests conducted in dry air, on chalcopyrite sample A, milled for 
15, 30, 60, and 120 min, are shown in figure 4. These tests again 
illustrate that the ignition point decreases with decreasing particle 
size. Two additional chalcopyrites, samples B and C, were tested 
and their ignition points are also listed in table 4. When this sulfide 
was oxidized in moist air, the samples gained weight prior to igni- 
tion. Water vapor did not change the ignition point for samples A 
and B but did raise the ignition point of sample C. XRD analysis 
of chalcopyrite samples oxidized to a final temperature of 450° C 
indicated the presence of ferric oxide, cupric sulfate, and cupric 
oxide. 



CHALCOCITE 

Chalcocite is also an important ore of copper and it has the 
chemical formula Cu 2 S. Figure 5 shows TGA results of chalcocite 
sample A milled for 15 and 120 min and oxidized in dry air. In- 
stead of the typical weight loss, this figure illustrates a sample weight 



o 
o 
<j 

< 

X 



120 



no 



100 



90 





1 1 1 

Chalcocite A 


1 


1 f 
Bl 


- 


KEY 




/ ; 




A 15 min 




1*1 




B 120 min 




U 


1 1 1 1 1 



100 



150 



200 250 300 

TEMPERATURE, °C 



350 



400 



Figure 5.— Thermal gravimetric analysis of chalcocite sample 
A milled for 15 and 20 min and oxidized with air. 



58 



gain. Sulfate formation would cause a weight gain, and the presence 
of cupric sulfate in the oxidized sample was confirmed by XRD 
analysis. As cupric sulfate formation is an exothermic reaction, and 
as figure 5 indicates a weight change rate similar to that noted in 
previous tests, the energy produced would be similar to that of ig- 
nition to an oxide. 

The standard heats of reaction of chalcocite and other com- 
mon sulfides to form oxides and sulfates are listed in reference 4. 
Table 4 lists the temperature at which rapid weight gain was in- 
itiated for the two chalcocite samples. In this group chalcocite A 
milled for 120 min reacted at the lowest temperature (335 ° C). The 
addition of water vapor to the oxidizing gas increased the sample 
weight slightly in the early stages of oxidation and it tended to raise 
the temperatre at which rapid weight gain occurred. 

GALENA 

Galena (PbS), a lead sulfide, is the most common ore of lead. 
TGA of samples milled for 120 min failed to exhibit the rapid weight 
change observed with previous sulfides and no ignition was noted 
at a final test temperature of 500° C. The addition of water vapor 
caused the sulfide to undergo a slight weight gain during heating 
and XRD analysis of samples oxidized isothermally at 300 ° C con- 
firmed that lead sulfate was formed. 



SPHALERITE AND WURTZITE 

Sphalerite and wurtzite are both zinc sulfides having the same 
chemical formula (ZnS), but with different crystal structures. TGA 
tests conducted on the zinc sulfides to a final temperature of 500 ° 
C gave no indication of a rapid weight change reaction (ignition). 
The finest milled wurtzite underwent about a 1-pct weight gain dur- 
ing oxidation with air containing 40 pet water vapor. This weight 
gain was initiated at about 450° C. 



MOLYBDENITE 

The final sulfide examined in this series was molybdenite, which 
has the chemical formula MoS 2 . The results of TGA tests conducted 



UJ 

o 

CD 



105 



100 



95 



90 - 



85 

100 150 200 250 300 350 400 450 500 

TEMPERATURE, °C 

Figure 6.— Thermal gravimetric analysis of molybdenite milled 
for 120 min. 



on this sulfide, milled for 120 min and oxidized with air and with 
air containing 40 pet water vapor, are shown in figure 6. The 
presence of water vapor failed to alter the shape of the weight-change 
curve prior to ignition and only lowered the ignition point slightly 
from 375° to 370° C. Additional ignition point values are listed 
in table 4 and indicate little effect due to particle size or oxidizing 
atmosphere. An interesting sidelight of the TGA tests was the for- 
mation of needlelike crystals of molybdenum oxide in the cool zone 
of the furnace tube. Molybdenum disulfide begins to sublime at 
450 ° C (5) and conducting tests to a final temperature of 500 ° C 
apparently produces sulfide vapors that are oxidized to M0O3, which 
in turn condenses to form crystals. 



1 1 1 1 1 
Molybdenite 


i 


i 






\B 


KEY 


A\ 




A Air 






3 Air containing 40 pet 






water 

i i i i i 




i 



RESULTS OF ISOTHERMAL OXIDATION ANALYSIS 



PYRITE 

Several of the sulfides that were tested by TGA underwent a 
weight gain prior to ignition. This weight gain was caused by the 
formation of sulfates, which is an exothermic reaction that could 
lead to spontaneous combustion fires. In an effort to obtain addi- 
tional information on this reaction at lower temperatures, the six 
pyrite samples examined by TGA were also oxidized isothermally 
and then subjected to a chemical analysis procedure to obtain a quan- 
titative measure of sulfide oxidation to sulfate. This method was 
found to be more convenient than gravimetric methods for conduct- 
ing long-term experiments. It also avoided the need to reconcile 
sulfide weight changes that are due to the volatilization of sulfur 
and the formation of sulfate and sulfur dioxide. 



Figure 1A illustrates the conversion of the pyrite to a soluble 
iron salt after oxidation at 200° C for 24 h in dry air and in air 
containing water vapor. These results illustrate the effect a small 
percentage of water vapor has on promoting the formation of water- 
soluble ferrous sulfate at this temperature. It should be noted that 
pyrite sample D ignited during the test because the self-heating due 
to sulfate formation raised the temperature of the sample. Because 
this sample burned to ferric oxide, it was not analyzed. A similar 
test series was run at 100° C and these results are illustrated in 
figure IB. Comparing the results, oxidation with air containing 5 
pet water vapor produced slightly more sulfate than dry air. Rais- 
ing the water vapor content to 60 pet promoted the oxidation of 
sulfide to sulfate with sample D undergoing a 30-pct conversion 
in 24 h. 



59 




O 
CO 



CO 

cr 

UJ 

> 
z 
o 
o 




Dry ai r 



Air with 5 pet 
water vapor 



Air with 60 pet 
water vapor 



Figure 7.— Isothermal oxidation of pyrite samples (A-F) to soluble iron at (A) 200° C and (B) 100° C. 



60 



CONCLUSIONS 



TGA was used to measure the reactivity of 10 types of metal 
sulfides in terms of the temperature at which the samples under- 
went the rapid weight change associated with ignition or sulfate 
formation. For the six pyrite samples there was a 40° to 50° C 
range in the ignition points. Attempts at associating the variability 
in ignition point with minor differences in the chemical or XRD 
analyses was not conclusive. Although each pyrite sample was 
milled in the same manner, it is possible that differences in the grind- 
ability of the pyrites could result in slightly different particle size 
distributions that could influence reactivity. Research to resolve the 
factors that cause one sample to exhibit a greater reactivity than 
another is continuing. 

Adding water vapor to the oxidizing atmosphere was found to 
raise the ignition point for all the pyrites. The addition of water 
vapor also caused the pyrites to gain weight prior to ignition. 
Chemical and XRD analysis of partially oxidized samples confirmed 
that the weight gain was due to the formation of ferrous sulfate. 
As this is an exothermic reaction it is quite probable that sulfate 
formation can lead to the self-heating that causes sulfide ignition. 

Pyrrhotite is often suspected as the mineral responsible for 
sulfide fires, however, the pyrrhotite samples examined in this report 
had fairly high ignition points. This was not unexpected as Good 
(J) reported a wide variation in ignition point values for samples 
taken from an ore pillar with known self-heating tendencies. Water 
vapor in the oxidizing atmosphere had a small effect on the pyr- 
rhotite ignition points determined by TGA and had a mixed effect 
on sulfate formation. 

The ignition point measured for the finely milled marcasite, 
when oxidized in moist air, was the lowest of all the sulfides tested. 
While water vapor had the effect of raising the ignition point of 
pyrite, it lowered the value measured for marcasite. Water vapor 
also promoted the formation of sulfate during the oxidation of mar- 
casite. Arsenopyrite ignited at about the same temperature as pyr- 
rhotite and like marcasite its ignition point was lowered when water 
vapor was present. Water vapor also promoted the weight gain of 
this sulfide at lower temperatures. 

The ignition points of the chalcopyrite samples were in- 
termediate to the common iron sulfides of pyrite and pyrrhotite. 
The presence of water vapor tended to promote sulfate formation 
but it had a mixed effect on the ignition point. Chalcocite failed 



to ignite during TGA testing, instead it underwent a rapid weight 
gain that was caused by the formation of cupric sulfate. As the heat 
produced from this exothermic reaction would be similar to igni- 
tion, the temperature at which it occurred was noted. These values 
were similar to the ignition points measured for chalcopyrite. Water 
vapor in the oxidizing atmosphere had a slight effect in promoting 
sulfate formation in the early stages of oxidation of chalcocite but 
tended to raise the temperature at which rapid weight gain occurred. 
The lead and zinc sulfide samples failed to ignite below 500° C 
and because of their low reactivity little sample weight gain was 
noted at lower temperatures. Molybdenite ignited at about the same 
temperature as the copper sulfides but did not exhibit the weight 
gain prior to ignition that was noted with many of the other sulfides. 
While the ignition point of the iron and copper sulfides decreased 
as the particle size decreased, the ignition point of molybdenite was 
not affected by particle size. 

Based on ignition point, several of the pyrites and the mar- 
casite sample were found quite reactive. The remaining iron and 
copper sulfides were found to have slightly higher ignition point 
values but like pyrite and marcasite may form sulfates at lower 
temperatures that could lead to self-heating. The four iron sulfides 
(pyrite, marcasite, pyrrhotite, and arsenopyrite) have all been men- 
tioned as being responsible for initiating spontaneous combustion 
in underground mines (7) and in certain mines, sulfide samples could 
be found that are much more reactive than those studied in this 
report. The lead, zinc, and molybdenum sulfides tested were less 
reactive and by themselves should not present a spontaneous com- 
bustion hazard. 

Isothermal oxidation followed by chemical analysis was found 
to be a useful method to quantitatively measure the conversion of 
pyrite to sulfate at lower temperatures. This method illustrated the 
effect that water vapor had on promoting this exothermic reaction 
when oxidizing pyrite at 100° and 200° C. One of the more reac- 
tive pyrite samples ignited during testing at 200° C and this in- 
dicated self-heating to the ignition point when sulfate was formed. 
TGA testing uses a very small sample and the heat produced dur- 
ing sulfate formation is dissipated, however, when a larger sample 
was used the heat produced exceeded the heat that was dissipated 
and the sulfide in the combustion boat reached the ignition point. 



REFERENCES 



1. Ninteman, D. J. Spontaneous Oxidation and Combustion of Sulfide 
Ores in Underground Mines: A Literature Survey. BuMines IC 8775, 1978, 
36 pp. 

2. Farnsworth. D. J. M. Introduction to and Background of Sulphide 
Fires in Pillar Mining at the Sullivan Mine. CIM Bull., v. 70, No. 782, 
June 1977. pp. 65-71. 

3. Good, B. H. The Oxidation of Sulphide Minerals in the Sullivan Mine. 
CIM Bull., v. 70, No. 782, June 1977, pp. 83-88. 



4. Reimers, G. W., and K. E. Hjelmstad. Analysis of the Oxidation 
of Chalcopyrite, Chalcocite, Galena, Pyrrhotite, Marcasite, and 
Arsenopyrite. BuMines RI 9118, 1987, 16 pp. 

5. Killeffer, D. H., and A. Linz. Molybdenum Compounds— Their 
Chemistry and Technology. Interscience Publ., 1952, 407 pp. 



61 



EMISSION PRODUCTS FROM WOOD CRIB AND 
TRANSFORMER FLUID FIRES 



By Margaret R. Egan 1 



ABSTRACT 

The Bureau of Mines investigated the characteristics of the combustion products of 
wood cribs and transformer fluid. The products from these two diverse fuels were analyzed 
for gas production and smoke characteristics. This included the combustion of four wood 
crib configurations at several ventilation rates and three commercially available brands of 
transformer fluid. 

Each fuel was studied independently and the results compared. These studies indicate 
that wood crib fires may be more difficult to detect because they are cleaner burning (as 
measured by the smoke obscuration level). By comparison, transformer fluid fires produce a 
thick smoke that reduces visibility, making escape and rescue more difficult. 



INTRODUCTION 



The Bureau of Mines conducts research to improve health 
and safety conditions in the mining industry. Exceptional 
circumstances, such as an underground fire, create life- 
threatening situations. Escape is dependent upon early detec- 
tion of a fire, and is hampered by reduced visibility due to 
smoke and the toxicity of the combustion products. In order to 
design more efficient detection and rescue equipment, the 
Bureau has investigated emission products of combustible 
materials found in underground mines. Similar analyses could 
also be beneficial in determining the existence, stage, or extent 
of an underground fire. 

These two fuels investigated were chosen either because of 
the quantities used or their potential hazard. Wood is the most 



abundant material found in mines. It has many uses especially 
as supports for mine workings. Transformer fluid is the 
coolant used in electrical transformers. Fires and explosions 
are potential risks whenever petroleum products are part of 
electrical equipment. 

The objectives of this study were to analyze and compare 
the combustion emission products of these two fuels. Com- 
parisons of heat-release rates, smoke particle sizes, smoke 
obscuration levels, and smoke and gas production constants 
are included. These measurements form a data base with 
which results of future studies of other mine combustibles can 
be compared. 



EXPERIMENTAL EQUIPMENT 



INTERMEDIATE-SCALE FIRE TUNNEL 

The Bureau's intermediate-scale fire tunnel was used to 
simulate a mine environment. This tunnel has been shown to 
successfully predict full-scale mine conditions. 2 A schematic 
of the tunnel with its data acquisition system is shown in figure 
1. The tunnel is 0.8 m wide by 0.8 m high by 10 m long and is 
divided into several sections. The first horizontal section is 1.5 



1 Research chemist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, 
PA 

2 Lee, C. K., R. F. Chaiken, J. M. Singer, and M. E. Harris. Behavior of 
Wood Fires in Model Tunnels Under Forced Ventilation Flow. Tests with 
Untreated Wood. BuMines Ri 8450, 1980, 58 pp. 



m long and cone shaped. It is hinged and can be lifted to allow 
entrance for the placement of the fuel. It begins with an 
air-intake cylinder that is 0.25 m long by 0.3 m in diameter and 
gradually enlarges until it matches the tunnel dimensions at the 
hinged area. Next is the fire zone where a gas burner is located 
and the fuel pan is balanced on a load cell. The fire zone and 
the remaining horizontal section are lined with firebrick and 
contain thermocouples, flow probes, and gas sampling ports. 
The diffusing grid begins the vertical exhaust section of the 
tunnel. Located in this section is an orifice plate that can be 
manually adjusted to attain the desired airflow. The final 
section of the exhaust section is horizontal and ends with an 
exterior fan. 



62 



22 m 




10- m length 



0.8 -m square duct 



intake 



Load eel 



I2 _ m length 
0.6l - m diam duct 



-0.305-nrdiam 
entrance duct 
hinged and movable) 




Manually 
adjustable 
orifice plate 



' — Diffusing grid 



Air 



TP 



'mi 



1 exhaust 
Ventilation 

fan 
( 2-speed ) 



TEOM 



CNM 



CO meter 



CO2 meter 



Pressure transducers 



LLLJ 



48" 
channel 
data- 
acqui- 
sition 
system 



3X 
- detector 



-Load cell 

-Digital input 

for CNM 

range 



DECNETn 



PDP 
11/44 



Control 
terminal 



\ . 



VAX 

11/780 



Printer 



VAX 

terminal 



■•• • 2o • ••• 
thermocouples 



A Pressure transducer (flow probe) 

x Differential pressure transducer 

* 3\ detector 

• Thermocouples 
■ Sampling ports 



CALCOMP 
plotter 



KEY 

CALCOMP California computer products 

CNM Condensation nuclei monitor 

DECNET Digital equipment networking 

PDP Programmed data processor 



VAX 



Virtual address extension 



Figure 1 .—Schematic of intermediate-scale fire tunnel (top) and data-acquisition system (bottom). 



DATA LOGGING 

The tunnel was equipped with a 48-channel data collec- 
tion system. As the experiments were in progress, all channels 
were scanned, recorded, and calculated. 



WOOD CRIBS 

Four configurations of wood cribs were tested in dupli- 
cate. The height and spacing of the sticks were chosen to study 
the growth and propagation of the fire. The cribs were 



constructed of Douglas fir using a small amount of carpenter's 
glue for joining. The configurations and their dimensions are 
shown in figure 2. 



TRANSFORMER FLUID 

Ten experiments were completed using three commercially 
available brands of transformer fluid — four with brand A and 
three with brands B and C. A 25-cm-diam pan filled with 
transformer fluid to a depth of 2.5 cm was used for each 
experiment. 



63 



3.01625 cm 




29.21cm 




29.21 cm 



Standard high 



Quadratic 





29.21 cm 



Standard low Linear 

Figure 2.— Crib configurations. Air flows from left to right. 

INSTRUMENTATION 



All instruments were periodically cleaned and calibrated 
according to manufacturers' instructions for the quantity of 
smoke or gas and the amount of use each had received. 

GAS MONITORS 

The CO analyzer measures accurately within 1 pet of full 
range or ±5 ppm. The C0 2 analyzer measures accurately 
within 1 pet of full range or ±250 ppm. These analyzers were 
calibrated at the beginning of each experiment. In addition, 
the concentrations of the span gases were checked at the 
beginning of each series of experiments. 

SMOKE MONITORS 

The particle number concentrations (N ) of the smoke 
particles were obtained by a condensation nuclei monitor, 
manufactured by Environment One Corp. 3 In the range of 



these experiments, the calculated error including the dilution 
factor was ±22 pet. 

The particle mass concentration (M ) of the smoke was 
obtained by a tapered-element oscillating microbalance, devel- 
oped by Rupprecht and Patashnick Co., Inc. It is capable of 
measuring dust concentrations with a better than 10 pet 
accuracy at the 250-ftg/m 3 level. 

A three-wavelength light transmission technique was used 
to measure particle size and light obscuration. This technique 
was developed by the Bureau. 4 

WEIGHT-LOSS MONITOR 

Continuous weight loss information was obtained by a 
strain-gauge conditioner and a load cell with a range up to 
22.68 kg. Their combined accuracy is stated as 0.05 pet of full 
scale or ±11.3 g. 



3 Reference to specific products does not imply endorsement by the Bureau of 
Mines. 



Cashdollar, K. L., C. K. Lee, and J. M. Singer. Three-Wavelength Light 
Transmission Technique To Measure Smoke Particle Size and Concentration. 
Appl. Opt., v. 18, No. 11, 1979, pp. 1763-1769. 



64 



TYPICAL TEST PROCEDURE 



The pan containing the fuel was positioned inside the 
tunnel. The shaft of the pan extended through a hole in the 
tunnel floor and was supported on the load cell. Prior to each 
experiment, background readings were obtained after the fuel 
was positioned and the exhaust fan started. All instruments 
were continuously scanned and data were recorded throughout 
the experiment. 

The fuel was ignited with a natural gas burner located 
immediately upstream from the pan. Once ignited, the fuel 
rapidly reached a steady-state, flaming stage. Once the flames 
are no longer visible, the decaying or smoldering stage begins. 

At a ventilation rate of approximately 0.65 m 3 /s, the 
transformer fluid took less than 1 min to ignite. The ventila- 



tion rate was then lowered to approximately 0.47 m 3 /s during 
the steady-state burning. The flames engulfed the entire pan 
instantaneously. As the transformer fluid was consumed, the 
flames began to die down. The experiments were concluded 
when the flames were no longer visible. 

The ignition time of the wood was longer (at least 3 min) 
depending on the crib configuration. Most cribs were burned 
at a ventilation rate of approximately 1 m 3 /s. They burned 
rapidly for at least 7 min and smoldered for an additional 30 
min. The experiments were concluded when the CO concen- 
tration returned to background level. 



CALCULATIONS 



It is necessary to measure certain parameters in order to 
compare the steady-state combustion products and ultimately 
the hazards of various fuels. Among these are gas concentra- 
tions, smoke particle mass and number concentrations, venti- 
lation rate, and mass loss rate. Other combustion properties 
can be calculated once these values are known. 



PRODUCT GENERATION RATES 

The generation rate (G x ) of a gas is related to its density, 
ventilation rate, and concentration by the expressions 



and 



where M c 



M r 



x ACO, 



x V A x ACO, 



(1) 
(2) 



and 



V A 
AX 



= 1.97 x 10- 3 g/(m 3 ppm), 

= 1.25 x 10- 3 g/(m 3 ppm), 

= ventilation rate, m 3 /s, 

= measured change in a given gas, ppm. 



Substituting these values in equation 3 yields 
Q A = V A [(0.01875)(ACO 2 ) + 6.06 X 10" 3 (ACO)] (4) 

for wood and 

Qa = V A [(0.025 l)(ACO z ) + 7.07 x 10" 3 (ACO)] (5) 

for transformer fluid. Because measurements of V A , AC0 2 , 
and ACO were made continuously, the actual heat release rates 
and gas generation rates could be calculated, using equations 1 
through 5. 

A typical fire rarely attains the state of complete combus- 
tion. Therefore, the actual heat of combustion (H A ) is usually 
less than the total heat of combustion. By calculating both the 
actual heat release rate, using equations 4 and 5, and measur- 
ing the fuel mass loss rate, M f , the actual heat of combustion 
can be calculated from the expression 



H A = Q A /M f . 



(6) 



HEAT RELEASE RATES 

It has been shown 5 that the total heat release rate realized 
during a fire can be calculated from the expression. 






H, 



Hco (K C o) 



K, 



(3) 



where Q A = actual heat release, kW, 

H c = net heat of complete combustion of the fuel, 
16.4 kJ/g for wood and 40.7 kJ/g for trans- 
former fluid, 
K co = theoretical yield of C0 2 , 1.723 g/g for wood 

and 3.19 g/g for transformer fluid, 
H co = heat of combustion of CO, 10.1 kJ/g, 
and K co = theoretical yield of CO, 1 .097 g/g for wood and 
2.03 g/g for transformer fluid. 



5 Tewarson, A. Heat Release Rate in Fires. Fire and Mater, v. 4, No. 4, 1980, 
pp. 185-191. 



In a flaming fire, the actual heat release rate can be used to 
estimate the fire hazards. However, in an actual mine fire, it is 
difficult, if not impossible, to measure the fuel mass loss. 
Therefore, the actual heat of combustion cannot be calculated. 
Since the true yield of a combustion product depends upon 
this information, significant errors can result in predicting the 
resultant concentration increases. 



PRODUCTION CONSTANTS 

The generation and heat release rates can be used to 
calculate production constants, or beta values (/3 X ), by the 
expression 



0x = Gx/Q, 



(7) 



Once beta values have been determined for a given fuel, the 
resultant gas and smoke concentration can then be calculated 
as a function of the ratio of fire size to ventilation rates. 



65 



SMOKE PARTICLE DIAMETERS 

Measurements of both number and mass concentrations 
of the smoke can be used to calculate the average size of the 
smoke particles by the expression 



7rd r 



N„ = 1 x 10 j IvL 



(8) 



where 



p p = individual particle density, g/cm 3 , 
d m = diameter of a particle of average mass, fim, 
and 1 x 10 3 = the appropriate unit's conversion factor. 
Assuming a value of p p = 1.4 g/cm 3 , then the diameter of 
average mass can be calculated from 



d m = 11-09 (£ 



(9) 



when the particle diameter is expressed in micrometers. 

Another method was used to determine the size of the 
smoke particles. It uses the three-wavelength smoke detector. 



Light was passed through the smoke and transmission of light 
(T) was calculated for each wavelength. An extinction- 
coefficient ratio can be calculated for each pair of wavelengths 
(X) by the following log-transmission ratios: 



InT(M.OO) lnT(Al.QO) 
lnT(\0.63) ' lnT(X0.45) 



or 



lnT(A0.63) 
lnT(\0.45) 



Using these extinction coefficients and the Cashdollar curve, 6 
the mean particle size (d 32 ) can be determined. (Calculation of 
the extinction-coefficient curves assumes spherical particles 
with an estimated refractive index.) 

Smoke obscuration is the percentage of light absorbed by 
the smoke or 100 pet of the light minus the percent transmis- 
sion. It is calculated using the following equation: 



Obscuration = 100(1 - T). 



(10) 



The obscuration rate is an average of the attenuation of the 
beam of light at the two wavelengths in the visible range, 0.45 
and 0.63 /xm. 



RESULTS OF COMBUSTION OF WOOD CRIBS 



The wood cribs burned in three distinctive stages: igni- 
tion, flaming, and decaying. Table 1 lists the length of the 
ignition stage and the steady-state burning, the mass loss rates 
for each stage, and the ventilation rates for each experiment. 
Steady-state burning refers to the rapidly burning period. The 
mass loss rate is the rate at which the wood was consumed. The 
length of the ignition stage indicates the ease with which each 
configuration was ignited. It was dependent upon the place- 
ment of the sticks for heat transfer and their accessibility to the 
burner flames. 

All the values listed in this paper are an average of the 
steady-state flaming stage. It was in this stage that the greatest 
mass loss, heat, and C0 2 were produced. Following steady- 
state burning is the decaying or smoldering stage in which most 
of the CO was produced. At this time, 84 pet of the available 
wood had been burned and only a low-level postflame smol- 
dering remained. 

Graphs in figure 3 compare CO versus C0 2 concentra- 
tions, heat-release versus total mass loss, particle mass versus 
number concentrations, and show the diameter of average 
mass as a function of time for test 3. In this experiment, the 
standard high crib ignited at 4 min at which time the burner 
was turned off. 



GAS CONCENTRATIONS AND HEAT 
PRODUCTION 

Table 2 lists the gas concentrations, generation rates, and 
heat production for each crib configuration. The high produc- 
tion of C0 2 indicated that these wood cribs were burning in an 
oxygen-rich environment. The reaction favored complete com- 
bustion. An average concentration of 6,760 ppm was produced 
during the flaming stage when the wood was being consumed 
at its maximum rate. The average generation rate for C0 2 was 
calculated to be 11.5 g/s. 

The CO concentration averaged 145 ppm during the 
flaming stage. Because CO is a product of incomplete com- 
bustion, it reaches its highest concentration in the decaying 



Table 1. — Ignition times, steady-state burning times, 
ventilation rates, and mass loss rates for wood cribs 

Crib type ] f mn Steady-state Venti | ation , Mass loss rate, g/min 

and test time ' burmn 9 time . m 3 /s 

min min Ignition Flaming Decaying 

Standard low: 

Test 2 8.1 15.3 1.0525 28.9 448.86 13.96 

Test 4 14.68 15.62 1.0363 17.65 412.96 26.65 

Quadratic: 

Test 1 6.42 10.08 1.0279 19.9 647.5 17.3 

Test 8 9.23 15.7 .8211 15.41 529.87 5.75 

Standard high: 

Test 3 3.6 10.8 1.0471 38.31 725.35 5.7 

Test 7 4.6 12.0 .4859 63.51 668.14 16.72 

Linear: 

Test 5 9.75 8.92 1.0018 35.38 778.62 5.59 

Test 6 6.0 6.7 .6167 48.99 769.68 13.43 



Table 2. — Gas concentrations, generation rates, 

and heat production for wood cribs 

Crib type C0 2 , G C02 , CO, G co , Q A , 

and test ppm g/s ppm 10" 2 g/s kW 

Standard low: 

Test 2 4,948 10.26 102 13.42 98.2 

Test 4 4,321 8.82 119 15.41 84.6 

Quadratic: 

Test 1 ND ND 143 18.37 ND 

Test 8 5,921 9.58 162 16.63 91.9 

Standard high: 

Test 3 7,215 14.88 204 26.70 142.8 

Test 7 3,446 3.30 120 7.29 31.7 

Linear: 

Test 5 10,158 20.05 143 17.91 191.3 

Test 6 11,303 13.73 163 12.57 131.1 

ND Not determined. 



H A . 
kJ/g 



13.1 
12.3 



ND 
10.4 



11.8 

2.9 



14.7 
10.2 



Work cited in footnote 4. 



66 



400 




14 


90 


12 


80 




70 


10 




E 

CL 


■° E 60 


o 


•v 


I 1 50 


6 c\j 


£ 40 


o 


< 


u 


s 30 


4 






20 


2 






10 








8 


0.35 



o 



co 

CO 
O 



CO 
CO 
< 





1 


1 


1 ' ' c 


- 






— 


- 


1 If 
A i ' 




KEY 
Mass 


/'l 
1 1 


1 1 ' V 

\ V V 




Number 


1 1 
1 
— \f* 




\\ '\ 


A 




, / 


V' i 






, 1 
I f 

1 ' 

\ 1 




AI'V 
\ \ 

\ *■ 
/ \ N 
i \ \ 


f 


\l 




\ 1 \ v 


*\ 






\ / \ N 


1 






\J ^s~\ \ _ 


1 


1 


1 


i V 



20 
18 

14 | 

a. 

12^ 
o 

10 ~ 
cr 

o UJ 
O CD 

6 3 

z 
4 

2 






Figure 3 
C, particle 



TIME, mm 

—Results of test 3, standard high crib. A, Heat-release rate and mass loss rate; B, CO and C0 2 concentrations; 
mass and number concentrations; and D, diameter of average mass. 






stage. Approximately 21 min after the crib ignited, the average 
maximum concentration of 378 ppm was achieved. Generated 
CO levels, at this time, averaged 0.39 g/s. Figure 3A shows the 
actual gas concentrations for test 3. A shift of the peak 
concentration for each gas is clearly visible. 

The concentrations of CO and C0 2 measured during the 
flaming Fire can be used to calculate the fire size, provided the 
ventilation in the affected entry is known. Remote sensing of 
these two gases could contribute significant information about 
the intensity and stage of a mine fire. In these experiments, the 
average fire size or heat release was 110.2 kW. 

Figure 35 shows the relationship of the heat-release rate to 
the mass loss rate. As the majority of the wood is consumed, 
the greatest heat is released. 



SMOKE CHARACTERISTICS 

The average number of smoke particles (N ) produced 
during the flaming stage was 6.0 x 10 6 p/cm 3 . The average 
particle mass concentration (M ) produced during the flaming 
stage was 49 mg/m 3 . In most experiments, the production of 
smoke dropped as the flaming stage was ending but increased 
slightly during the decaying stage. Figure 3C shows the actual 
data for test 3 and table 3 lists the smoke characteristics for 
each experiment. 

The size of the smoke particles showed the same variation 
as the smoke production. The largest particles (averaging 0.22 
/*m) were produced in the flaming stage during their highest 
production rate. Figure 3D shows the data for test 3. The 



67 



Table 3.— Smoke characteristics for wood cribs 

Crib type N , M , d m , Obscuration rate, 

and test 10 6 p/cm 3 mg/m 3 ^m pet 

Standard low: 

Test 2 5.26 82.84 0.278 2.1 

Test 4 7.20 31.40 .181 13.0 

Quadratic: 

Test 1 1.13 ND ND 12.0 

Test 8 8.09 84.99 .243 5.2 

Standard high: 

Test 3 9.06 40.90 .183 6.0 

Test 7 3.24 33.38 .241 9.8 

Linear: 

Test 5 9.02 38.62 .180 13.1 

Test 6 5.31 31.70 .201 7.9 

ND Not determined. 



Table 4.— Production constants for wood cribs 

Crib type /3 CC , 2 , /3 co , /3 No , Mo , 

and test 10^ 2 g/kJ 10' 3 g/kJ 10'° p/kj 10~ 4 g/kJ 

Standard low: 

Test 2 10.45 1.37 5.64 8.88 

Test 4 10.42 1.82 8.82 3.85 

Quadratic: 

Test 1 ND ND ND ND 

Test 8 10.43 1.80 7.23 7.60 

Standard high: 

Test 3 10.42 1.87 6.64 3.00 

Test 7 10.40 2.29 4.96 5.11 

Linear: 

Test 5 10.48 .94 4.72 2.02 

Test 6 10.48 .96 2.50 1.49 

ND Not determined. 



quantity, mass, and size showed a slight increase during the 
decaying stage. The average diameter of mass during the 
decaying stage was 0.16 ^m. 

PRODUCTION CONSTANTS 

Table 4 lists the production constants for all wood exper- 
iments. Crib configuration seemed to be of more importance 
than the ventilation rate. The standard high and linear cribs 
tended to have lower smoke production constants than the 
other configurations. They were also the fastest burning cribs. 
The linear crib also had the lowest CO production constant 
because more complete combustion occurred during its short 
burning time. The C0 2 production constants are all approxi- 
mately the same indicating the even production regardless of 
the fire size. 

CRIB CONFIGURATION 

As can be seen in table 1 , the configuration of the crib was 
a significant factor in the ignition time, the length of the 
burning time, and the mass loss rate. The rate at which any fire 
propagates largely depends upon the heat transfer. When the 
arrangement of the sticks and the ventilation rate was varied, 
a noticeable difference in the heat transfer occurred. The best 
configuration for fire propagation was the linear crib. Its stick 
arrangement permitted the most efficient heat transfer, which 
produced the greatest mass loss and heat release rates and 
shortest burning time. Its flaming stage lasted only 8 min. 

Lowering the profile of the crib decreased the mass loss 
rate and lengthened the time of ignition. The standard low crib 
had the least efficient heat transfer, resulting in the longest 
steady-state stage and lowest mass loss rate. 

Decreasing the angle at which the sticks were positioned 
increased the mass loss rate but did not affect the time of 
ignition. The quadratic and linear cribs ignited in about the 
same time, but the linear crib, with a lower angle and a more 
efficient heat transfer, burned faster. 



VENTILATION RATE 

Lowering the ventilation rate reduced the mass loss rate. 
With less available oxygen, the wood was consumed at a much 
slower rate. In addition, the efficiency of the heat transfer was 
reduced. In underground fires, it may be beneficial to decrease 
or stop the airflow, if possible. This would slow the progress of 
the fire by reducing the available oxygen and by lowering the 
heat transfer. The same relationship of burning rates to 
airflows has been reported. 7 However, the burning rates 
reached a maximum at airflows between 2 and 3 m 3 /s. Beyond 
this point, convective cooling competes with the heat transfer 
and slows the burning rate. 

Larger sized particles were produced at lower ventilation 
rates. The slower transport time may have permitted the 
particles to coagulate. Incomplete combustion may also have 
contributed to the production of larger sized particles. 

A representative sample of the combustion gases may be 
difficult to obtain with low ventilation rates. Stratification of the 
gas layers may have accounted for the low C0 2 concentration. 



BURNING RATE 

The burning rate affects the smoke particle characteris- 
tics. The rapid oxidation of the linear crib yielded a clean 
burning fire with less smoke and toxic gas production. Much 
higher production constants are generated from the slower 
burning standard low crib. The other smoke characteristics 
show little variation among the different configurations. The 
smoke obscuration must be at least 15 pet before the mean 
particle diameter (d m ) can be calculated. 



7 Tewarson, A. Analysis of Full-Scale Timber Fire Sets in a Simulated Mine 
Gallery. Factory Mutual Res. Corp., Norwood, MA, Tech. Rep. J. I. OEON1.RA 
and J. I. OFON3.RA, June 1982, 55 pp. 



bS 



RESULTS OF COMBUSTION OF TRANSFORMER FLUID 



All three brands showed similar results for gas production, 
heat release, and heat of combustion. However, the tested brands 
showed somewhat different results for smoke characteristics. 

GAS CONCENTRATIONS AND HEAT 
PRODUCTION 

The CO concentration remained fairly constant, rising 
slightly as the flames died. The C0 2 concentration gradually 
rose at an average rate of 22 ppm/min as the fuel was 
consumed. The CO and C0 2 concentrations for all brands are 
found in figures 44 and 4B. 

The heat release rate remained fairly constant for most of 
the experiments, increasing slightly just before the flames died. 
The average total mass loss was 2,000 g, at a rate of 0.87 g/s. 
The actual heat of combustion also remained fairly constant 
throughout the steady-state burning but rose sharply just 
before the fuel was completely consumed. The average values 
for each brand are listed in table 5. 

SMOKE CHARACTERISTICS 

The particle number concentration (N ) slowly increased 
until the fuel was almost completely consumed and then it 
started to drop. The particle mass concentration (M ) varied 
throughout the experiments. An average was taken during the 
steady-state burning when it was the most stable. Using these 
values, the diameter of average mass (d m ) was calculated. The 
average values for each brand are listed in table 6. The mass 
and number concentrations and the diameter of average mass 
for each brand are found in figure 5. 

Using the three-wavelength smoke detector, the average 
mean particle size (d 32 ) can be calculated for each wavelength 
These averages and the obscuration rate for each brand are 
listed in table 7. 



Table 5. — Gas concentrations and heat production 
for transformer fluid 



Brand 



CO, 

ppm 



C0 2 , 

ppm 



Qa. 

kW 



A 

B 

C 

Average . 



112 



1,779 



21.5 



Ha, 

kJ/g 



120 


1,682 


20.8 


24.1 


97 


1,871 


22.5 


25.5 


120 


1,784 


21.3 


24.9 



24.8 



Table 6. — Smoke characteristics for transformer fluid 



Brand 



N , 

10 6 p/cm 3 



M , 

mg/m 3 



d m , 



A 

B 

C 

Average . 



1.57 


40.6 


0.309 


1.35 


9.0 


.211 


.70 


58.1 


.501 



1.21 



35.9 



.340 



Table 7. — Mean particle sizes and obscuration rates 
for transformer fluid 



Brand 


lnT(X0.63), 
lnT(X0.45) 

/i(T! 


InT(XLOO), 
lnT(X0.45) 


lnT(X1.00), 

lnT(X0.63) 
/xm 


Average 
d 32 , 


Obscuration 
rate, 
pet 


A 

B 

C 


0.302 
.339 
.336 


0.351 
.395 
.431 


0.391 

.440 
.538 


0.348 
.391 
.435 


39.9 

40.8 
59.6 


Average 


326 


.392 


.456 


.391 


46.8 



1 60 



I40 



I20 



EIOO - 



o 
° 80 



60 - 



40 



20 




KEY 

Brand A 

Brand B 

— Brand C 



\ 

w 

I 



20 



30 



40 



50 




TIME, mm 
Figure 4.— Comparison of gas production for three brands of transformer fluid. A, CO concentrations; B, C0 2 concentrations. 



69 



70 



60 



50 



40 



30 



20 



10 



0.7 




'i 



KEY 

- Brand A 

- Brand B 

- Brand C 



_ I I 1 L 



L^ 






2.0 




1.8 




1.6 


■o 


1 4 


fc 








V 




Q. 


1.2 


o 




c r 


1.0 


y 




o 




( ) 




ir 


08 


UJ 




CD 




5 


0.6 


■z. 






0.4 




0.2 



rv\ 



v y V.' V V\ A< 



- \ 



10 20 30 

TIME, min 



40 



50 



Figure 5.— Comparison of smoke characteristics for 
three brands of transformer fluid. A, particle mass 
concentration; S, particle number concentration; and 
C, diameter of average mass. 



20 30 

TIME, min 



PRODUCTION CONSTANTS 

The production constants or beta values are calculated as 
a function of the fire size. For the tested brands of transformer 
fluid, the fire sizes are very similar. Therefore, it is expected 
that the beta values reflect the same variability as the gas and 
smoke concentrations. Table 8 lists the average production 
constants for each brand. 



Table 8.— Production constants for transformer fluid 



Brand 



0CO' /3c0 2 ' 0N O . 0M o i 

10~ 3 g/kJ 1CT 2 g/kJ 10 1o p/kJ 10" 4 g/kJ 



A 

B 

C 

Average 



3.43 


7.57 


3.59 


9.28 


2.51 


7.64 


2.80 


1.87 


3.24 


7.59 


1.51 


12.54 



3.06 



7.60 



2.63 



7.89 



COMPARISON OF TRANSFORMER FLUID BRANDS 



Brand B generated a slightly higher C0 2 concentration 
and lower CO concentration resulting in a larger fire size (Q A ) 
and actual heat of combustion (H A ). These analyses were 
based on relatively few experiments and may reflect only the 
range of gas production. Considering this, the gas concentra- 
tions generated by the tested brands of transformer fluid were 
similar. 



The differences between the brands are more evident in 
their smoke characteristics. The most noticeable variations are 
the 75 pet lower mass concentration of brand B and the 42 pet 
lower number concentration of brand C. These low values are 
also reflected in the reduced production constants. 

Because the particle size can be calculated by two inde- 
pendent methods, the diameters obtained by one method 



70 



should confirm those obtained by the other. The calculations 
indicate good agreement between the average d m and d 32 . The 
low number and high mass concentrations for brand C resulted 
in the largest calculated d m . By both methods, brand C 
produced the largest particles. This is corroborated by the high 
obscuration rate of brand C. 

However, differences are apparent in comparing the par- 
ticle sizes for brand B. The small mass concentration has 



lowered the d m , while the d 32 approximates the average. Here 
the agreement between the two methods is not very good. 

The results of these experiments showed little variation 
between the transformer fluid brands for CO and C0 2 pro- 
duction, heat release, and heat of combustion. However, the 
smoke characteristics data indicate that brand C produced the 
heaviest and thickest smoke, while brand B generally produced 
the lowest CO concentration. 



COMPARISON OF EMISSION PRODUCTS OF FUELS 



When the smoke characteristics from wood fires are 
compared with the smoke characteristics of transformer fluid 
fires, the data indicate that a wood fire must be five times 
larger to produce the same obscuration rate as that obtained 
for transformer fluid. Transformer fluid generates a denser, 
more hazardous, but more detectable fire than wood. 

When 84 pet of the light is obscured, reduced visibility 
lessens the possibility of escape. Using a 25-cm-diam pan, a 
46-pct obscuration rate was attained. This pool size is compa- 
rable to a very small spill. By comparison, the wood burns 
with very little light obscured. The particle sizes and obscura- 
tion rates for wood and transformer fluid fires are given in 
table 9. 

In addition to producing denser smoke, transformer fluid 
fires generate more toxic products than do comparably sized 
wood fires. Table 10 gives the emissions for fires at the same 
ventilation rates, when the transformer fluid fires equal the 
intensity of wood. Although transformer fluid smoke is more 
hazardous, its denser smoke may make it more detectable. A 
small transformer fluid fire may be detected before a well- 



developed wood fire. In addition, the speed with which liquid 
pool fires propagate makes early detection essential if life and 
property are to be protected. 



Table 9.— Wood and transformer fluid fire particle 
and obscuration rates 


sizes 


Fuel 


d m , 


d 32 , Obscuration 
/xm rate, pet 


Wood crib 


0.22 


ND 
0.39 


86 


Transformer fluid 


.34 


46.8 



ND Not determined. 



Table 10.— 


I/Vood and transformer fluid fire emissions 


Fuel 


CO, 

ppm 


co 2 , 

ppm 


No. 

10 6 p/cm 3 


M , 

mg/m 3 


Wood crib 


145 


6,759 
4,992 


6.04 
2.79 


49.1 


Transformer fluid... 


284 


85.2 



CONSIDERATIONS 



In any mine fire, all the burning materials combine to 
produce a wide variety of smoke particles and volatile gases 
that are transported by the ventilating system. A comparison 



of emission products is included as a means of demonstrating 
their hazardous characteristics and to assess the potential 
impact on the detectibility of the smoke produced. 



71 



APPENDIX.— SYMBOLS USED IN THIS PAPER 



u 32 

G x 
H A 
H c 

H co 

K x 

In 

M f 



conversion factor of a combustion product 

diameter of a particle of average mass, ^m 

mean particle size, /im 

generation rate of a given combustion product, g/s 

actual heat of combustion, kJ/g 

net heat of combustion of the fuel, kJ/g 

heat of combustion of CO, kJ/g 

theoretical yield of a given gas, g/g 

logarithm, natural 

fuel mass loss rate, g/s 

particle mass concentration, mg/cm 3 



M x 

N 

P 

Qa 
T 



AX 
X 



density of a given gas, g/(m 3 -ppm) 

particle number concentration, p/cm 3 

particle 

actual heat release, kW 

transmission of light 

ventilation rate, m 3 /s 

production constant of a given combustion product, 

g/kJ or p/kJ 

measured change in a given gas, ppm 

wavelength, /xm 

individual particle density, g/cm 3 



71' 



UTILIZATION OF SMOKE PROPERTIES 
FOR PREDICTING SMOKE TOXICITY 



By Maria I. De Rosa 1 and Charles D. Litton 2 



ABSTRACT 

The Bureau of Mines has conducted two series of experiments to determine if a smoke 
particle characteristic, such as the smoke particle of an average diameter, d g , average number 
concentration, n , and their product, d g -n , could be correlated with the smoke relative 
toxicity of smoldering combustibles. A previous set of experiments had shown that the smoke 
parameter d g -n differed among combustibles. 

In the first series of tests, the inverse of the smoke particle diameter-concentration 
product, l/d g n , yielded during the combustion of various combustibles, was found to 
correlate with smoke toxicity data found in the literature for similar materials. 

In a second, more detailed series of experiments, tests were conducted using samples of 
mine conveyor belts. For these belts, the main toxicity is HCl. The results of this second series 
of tests showed that the smoke parameter, l/d g n load, inverse of total d g n values per gram 
of sample weight loss, correlated directly with the HCl load, HCl concentration per gram of 
sample weight loss. 



INTRODUCTION 



In 1984, the Bureau of Mines initiated a series of experi- 
ments to study smoke particle characteristics, and to determine 
whether they differ among combustibles. In these tests, a wide 
range of combustibles including coal, wood, burlap, rubber, 
and polyvinyl chloride (PVC) brattice materials were tested. 
The results showed that the smoke particle of an average 
diameter, d g , smoke particle of an average number concentra- 
tion, n , and their product, d g -n , differed among combusti- 
bles (1-2). 3 The question posed was: does d g n differ accord- 
ing to the smoke relative toxicity of the combustion products? 

In the first series of tests of this study, attempts have been 
made to correlate the inverse of the smoke particle diameter- 
concentration product, l/d g n , yielded during the combustion 
of various materials, with relative toxicity data obtained 
during the combustion of similar materials, by Alarie (3) 
(animal toxicity data) and Paciorek (4) (chemical analyses), 
and reported in the literature. 

In the second series of tests, attempts have been made to 
correlate the smoke particle parameter, l/d g n load (inverse of 



' Industrial hygienist. 

2 Supervisory physical scientist. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 

'Italic numbers in parentheses refer to items in the list of references at the end 
of this paper. 



the total d g n values per gram of sample weight loss) with 
smoke relative toxic loads (toxic concentrations per gram of 
sample weight loss) evolved during the combustion of PVC, 
neoprene, and styrene-butadiene mine conveyor belts. 

Hydrogen chloride (HCl — threshold limit value, 5 ppm; 
short-term exposure limit, 25 ppm; immediately dangerous to 
life, 100 ppm) was found by Paciorek (4), and confirmed by 
the Bureau, to be the main toxic gas evolved at temperatures of 
<350° C (early combustion conditions, under which miners 
must plan and prepare for escape), during the combustion of 
all types of mine conveyor belts. The HCl release begins early, 
and proceeds rapidly at rates that depend on the chlorine 
content of the material, and on the ease with which materials 
undergo thermal decomposition (5). 

Paciorek (4), and confirmed by the Bureau, also found 
that PVC and neoprene belts released the highest concentra- 
tions, and styrene-butadiene the lowest. Other major toxic 
products, such as carbon monoxide (CO) was found (4) to 
evolve at much higher temperatures. 

Therefore, the study reported in this paper correlates the 
inverse of the smoke particle diameter-concentration product, 
l/d g n load, with the HCl loads yielded during the thermal 
degradation of three types of mine conveyor belts. 



73 



BACKGROUND 



Conveyor belts are complex mixtures of a variety of 
components. However, although the number of individual belt 
components is very large, the number of major pure compo- 
nents is comparatively small (6). The belting most widely used 
in deep mines contain either a halogenated base polymer (PVC 
and neoprene belts) or a halogenated additive (styrene- 
butadiene belts) added to impart fire resistance (7). Because of 
economic considerations, PVC resin probably is the most 
widely used of the halogenated polymers. However, this poly- 
mer releases HC1 at temperatures as low as 180° C, and the 
reaction is accelerated in oxidizing atmospheres; for pure resin, 
HC1 is the only product (90 pet) evolved at temperatures below 
200° C (8). Because PVC is a resin, it requires large quantities 
of plasticizers (phthalate esters) which co-evolve as phthalic 
anhydride, CO, and unsaturated hydrocarbons (9). Neoprene, 
because of its chlorine content, exhibits flame-retardant char- 



acteristics; however, although considered thermally stable up 
to 300° C, it has been found to degrade at 180° C; the onset 
temperature for cured compositions could be as low as 125° C 
(70). For neoprene, as for PVC, HC1 is the major degradation 
product at 250° C; it accounts for 80 pet of the chlorine 
present, with the rest liberated in the form of chlorinated 
materials (11). Styrene-butadiene raw gum is flammable, and 
requires large quantities of chlorinated additives, in addition 
to fillers, plasticizers, and sulfur organic activators (12); its 
final degradation products derive from all these constituents. 
The major toxic component is again HC1, although in much 
smaller quantities (13). Therefore, in the Bureau study, corre- 
lations have been established between HC1 loads and the 
inverse of the smoke particle diameter-concentration product, 
l/d g n load, evolved during the thermal degradation of mine 
conveyor belts. 



OXIDATIVE THERMAL DEGRADATION AND SUBMICROMETER 
PARTICLE DETECTOR-ANALYZER SYSTEMS 



The degradation system (fig. 1) consists of a furnace (9 by 
9 by 14 in) with a range of 100° to 1,200° C, set a priori at 
250° C for this study, which allows the temperature to rise 
automatically from ambient at a rate of 30° C/min to 40° 



C/min (fig. 2). During the experiments, the temperature is 
monitored continuously with type K thermocouples attached 
to a strip-chart recorder. 



Exhaust 



Lindberg furnace, 
9by9byl4in 



X 



Hood 



Exhaust 
Gas 
sampling 
port 



Thermo- 
couples 




Valve Quartz tubing, 

/ 5 /£Hn-diam by 3~ft long 



Temperature /\. 



Gas 
sampling 
port 

Quartz pedestal 
sample 



u u u 

Sampling ports 



control 



Load cell 

and 
accessory 




Bridge 
amplifier 



( Strip-chart recorders \ 
Weight loss Temperature 




Figure 1 .—Oxidative thermal degradation and submicrometer particle detector-analyzer systems. 



74 





350 




300 


o 







250 


«. 




Q. 




^ 




III 


200 


K 




rr 




hi 


IbO 


00 




^ 




< 


100 


o 





1 



Set furnace temp, 
250° C . 



50- 



1 r 



J L 



J L 



J L 



12 3456 789 10 
TIME, min 

Figure 2.— Furnace temperature versus time at 250° C. 



A universal load cell, located under the furnace floor and 
contacted by the furnace quartz sample-cup pedestal, trans- 
mits the sample weight loss to another strip-chart recorder. A 
pump draws ambient air continuously into the furnace (10 
L/min) via an opening at the center of the furnace door, and 
sends combustion air to the gas analyzers through a quartz 
tube (40 by 1 in) inserted in the upper right rear of the furnace. 
A flowmeter is installed between the pump outlet and the 
infrared gas analyzers for continuous visual flow indication. 
The furnace air is monitored continuously by two analyzers: 
one for CO (0 to 100 ppm and to 1 ,000 ppm) and the other 
for 2 (0 to 25 pet). 

The submicrometer particle detector-analyzer system (fig. 
1) consists of a strip-chart recorder and a submicrometer 
particle detector-analyzer (SPDA) through which a small 
quantity (1.6 L/min) of furnace airflow is directed. The SPDA 
is a prototype instrument developed by the Bureau; its basic 
operating principles are described elsewhere (14). The present 
version has been modified so that real-time data can be 
acquired for the simultaneous determination of the average 
smoke particle diameter and the average smoke particle con- 
centration, without need for the more time-consuming deter- 
mination of the actual distribution of particle diameters. 



DETERMINATION OF SMOKE RELATIVE TOXICITY AND CHARACTERIZATION 
OF SMOKE PARTICLE DIAMETER-CONCENTRATION 



In the first series of tests, the smoke toxicity data of 
various combustibles is derived from studies performed by 
Alarie (3) and by Paciorek (4). For Alarie, the toxic load of a 
specific combustible (LTC50) is derived from the quantity of 
material, and the time necessary to cause a 50-pct animal 
mortality. For Paciorek, the yields of toxic gases are deter- 
mined analytically in terms of quantity of toxic gas (toxic 
loads, parts per million per gram) produced per mass of 
sample consumed. The toxic load (TL) of a specific combus- 
tible is the sum of each toxic gas yield per 1 g of weight loss 
divided by its short-term exposure limit. To equate the data 
from these independent tests, the average TL value for wood 
reported by Alarie was normalized to the average TL for wood 
reported by Paciorek according to A/(LTC50) = (TL) (A = 
constant); for wood, A had a value of 4,600. All other LTC50 
values were scaled to toxic load according to TL = 
4,600/(LTC50). 

The TL values derived from Alarie data, via the preceding 
relationship, were in excellent agreement with the TL values 
derived analytically by Paciorek. 

The characterization of smoke particles during the com- 
bustion of similar materials (table 1), was done by obtaining 
the time-average value of the product d g n during the combus- 
tion, at 250° C set-furnace temperature, of materials similar to 
those used by Alarie and Paciorek. The inverse of this average 
value, l/d g n , was correlated with the relative toxicity (TL) 
obtained from references 3 and 4. 

Table 1.— Materials investigated during the first series of tests 

Material Description 

PVC brattice PVC yellow plastic, reinforced with nylon fabric. 

PVC resin PVC pellets. 

Wood Pine wood, untreated. 

Rubber Natural rubber mats. 

Cotton-polyester 65-pct-cotton, 35-pct-polyester fabric. 



In the second series of tests, the smoke relative toxicity 
(HC1) and the smoke particle diameter-concentration product, 
d g n , evolved during the combustion of mine conveyor belts 
(table 2) were derived in the following manner: sets of exper- 
iments (eight experiments in the set; each experiment repeated 



Table 2. — Materials investigated during the second series 
of tests 

Conveyor belt _ . .. „. , 

/ . i Description CU, pet 

material r d r 

PVC: 

P1 Polymer component is PVC resin with fillers. 18 

P2 do 16 

Neoprene: 

N1 Polymer component is neoprene rubber with 20 

fillers. 
N2 do 13.2 

Styrene- 
butadiene: 

S1 Polystyrene rubber, with chlorinated additives 2.3 

for fire retardancy. Contains carbon black, 
antiozonant, phosphorous, tackifying 
resins. 

S2 Polystyrene rubber, not treated for fire 1.25 

retardancy. Contains carbon black, 
antioxidant, antiozonant, phosphorous, 
tackifying resins. 

S3 Polystyrene rubber, with chlorinated 7.25 

additives. Contains carbon black, 
antioxidant, antiozonant, phosphorous, 
tackifying resins. 

S4 Polystyrene rubber, with chlorinated additives 15 

reinforced with nylon thread. Contains 
carbon black, antioxidant, tackifying 
resins. 



75 



three times) were performed at furnace set-temperature of 250° 
C for a duration of 14 min (these are conditions under which 
most of the HC1 evolves, with no gross combustion or 
oxidation of the sample) with 1 g sample of PVC (PI and P2), 
neoprene (Nl and N2), and styrene-butadiene (SI, S2, S3, and 
S4) conveyor belts. The variables measured as a function of 
time were the HC1 concentrations (parts per million) from 
which the toxic loads (HC1 concentrations measured at a 0.1 -g 
sample weight loss (= 10th minute) per 1 g of weight loss, 
parts per million per gram) were derived. 4 Other variables 
include the furnace temperature, the sample weight loss, and 
the SPDA voltages; the ratio of the experimental and initial 
SPDA voltage outputs (I e /I ) was use d to determine the 
product, d g -n (particles per square centimeter), of the smoke 
particle of an average diameter (d g , centimeters) and number 
concentration (n , particles per cubic centimeter), from which 
its inverse, l/d g n (square centimeters per particle), was 



derived, following the relationship in equation 1 (see also 
figure 3). 



1.. 



f = l/(Kd g n )(l-exp(-Kd g n )), 



(1) 



where K = charging constant (K = 0.012, cm 2 /p). 
The d g n load (total d g n values per 1 g of weight loss, particles 
per square centimeter per gram, was used to derive its inverse, 
l/d g n load, square centimeter per particle per gram). 

After each experiment, the furnace was turned off and the 
char was weighed. Measurements of HC1 concentrations were 
made at 2-min intervals at the sampling port nearest the 
furnace air exit using Draeger short-term exposure tubes. 
Analyses were performed to determine the chlorine content 
(percent) of each conveyor belt using the method of oxygen 
bomb combustion-ion selective electrode. 



RESULTS AND DISCUSSION 



First Series of Tests; Set-Furnace Temperature of 
250° C 

From figure 4, it is apparent that the parameter l/d g n 
correlates very well with the toxicity of the various combusti- 
bles. For each combustible material, the symbols represent the 
average of the data for l/d g n at the average value of the toxic 
load obtained from references 3 and 4. 

The rectangles around each symbol, along the abscissa, 
correspond to the total range of toxic loads derived from both 
studies, and along the ordinate, to the range of values of 
l/d g n obtained in the Bureau's series of tests for each type of 
combustible; the symbols represent the average values. A 
simple power curve fit to the data is defined by the solid line 
and is given by the expression (also shown in figure 4) RTH = 
2.303 x 10 5 (l/d g n ) 2 - 192 . The units of l/d g n represent some 
measure of the effective cross section for ionization of the 
different experimental smokes. This cross section also scales 
with the relative toxicity hazard or RTH. 

It could be argued that a high value of d g n signifies that 
a smaller percentage of the consumed mass is available for 
toxic gas production, while a lower value of d g n signifies that 
a higher percentage of the consumed mass is realized as toxic 
gas, rather than smoke. Although this argument agrees qual- 
itatively with the test results, no rigorous theoretical explana- 
tion is offered at this time. 



Second Series of Tests; Set-Furnace Temperature 
of 250° C 



In the second series of tests, the inverse of the smoke 
particle diameter-concentration, l/d g n load, correlated di- 
rectly and significantly (r = = 0.80) with the HC1 toxic loads 
for all types and kinds of conveyor belts (fig. 5). As expected, 




KEY 

* Rubber 
a Wood 

• Polyester 

a PVC brattice 
o PVC resin 
— 8est fit 
RTH = 2.303" 



J 12 192 



RELATIVE TOXICITY, RTH- 



4 Bureau-derived results are in agreement with experimental results obtained by 
Paciorek (4) with a different methodology. 



Figure 4.— Correlation of the inverse of smoke particle 
diameter-concentration product, 1/d g n ol with toxicity data from 
literature. 




Figure 3.— Smoke particle diameter-concentration relation- 
ship curve. 



io- 



5 1 0-3 



I0-4 



I0-5 





I 


I 




D _ J -^ < 


— 


Set-furnace temp, 250°C 






-Cr*. 


~ 


A ^^* 


O 

I 


• PI 
■ P2 

DN1 


KEY 

♦ N2 OS3 
ASl <>S4 
AS2 



10 100 

HC^ LOAD, ppm/g 



1,000 



Figure 5.— Correlation of the inverse of smoke particle diam- 
eter-concentration product, 1/d g n Q load, with HCI toxic loads 
during oxidative thermal degradation of conveyor belts, at 
250° C 



76 



PVC (PI and P2) and neoprene (Nl and N2) belts yielded the 
highest HC1 loads (maximum loads: 1,000 and 500 ppm/g), 
due to their high chlorine content, and the highest l/d g n load 
(0.0023 cm 2 /(pg)) (table 3), due perhaps to the fact that the 
majority of particles, being in the liquid state as HC1, are not 
being registered by the SPDA. By contrast, most of the 
styrene-butadience belts (SI, S2, and S3) yielded the lowest 



HC1 loads (20, 30, and 60 ppm/g) and the lowest l/d g n loads 
(0.000065 cm 2 /(pg)) (table 3), due to their low content of 
chlorinated additives. Exceptions were observed for belt S4, 
which yielded a much higher HC1 load (300 ppm/g) and a 
higher l/d g n load (0.00071 cm 2 /(p-g)), due to a higher 
percentage of chlorinated additives (table 3). 



Table 3.— Oxidative thermal degradation data at 250° C 

Time, min 

Conveyor belt 10th 14th 
material 1 

HCI, WL, 1/d % n c , HCI, WL, 1/d % n , 

ppm g cm /p ppm g cm /p 

PVC: 

P1, 18 pet Cl 2 220 0.2 0.016 370 0.4 0.0057 

P2, 16 pet Cl 2 363 .5 .007 500 .8 .0019 

Neoprene: 

N1,20pctCI 2 162 .3 .01 315 .7 .0034 

N2, 13.2 pet Cl 2 42 .1 .007 120 .36 .0012 

Styrene-butadiene: 

S1,2.3pctCI 2 .0013 2 .1 .00065 

52, 1.25 pet Cl 2 10 .4 .0003 18 .8 .0007 

53, 7.25 pet Cl 2 .0025 6 .1 .00096 

54, 15pctCI 2 28 .1 .019 70 .35 .0025 

WL Weight loss. 

1 1-g sample 

2 Derived from the 10th minute HCI concentrations per 1 gram of sample weight loss. 

3 Inverse of d g n loads. Each d g n load is the sum of all d g n values per gram of weight loss 





Derived loads 




2 HCI, 
ppm/g 


3 1/d g n ol 
cm 2 /(pg) 


d g n , 
p/(cm 2 -g) 


1,000 


0.0023 


440 


720 


.0015 


650 


540 


.0023 


430 


418 


.0005 


2,135 


20 


.000065 


15,500 


30 


.000053 


19,000 


60 


.000096 


10,400 


300 


.00071 


1,415 






CONCLUSIONS 



The results indicate that the smoke particle characteristic, 
d g n , is indicative of smoke toxicity by discriminating not only 
among materials whose main toxic loads are CO or HCI (first 
series of tests), but also among materials whose HCI loads 
varies from material to material (second series of tests). 
According to the findings, the inverse of the smoke particle 
diameter-concentration product, l/d g n 0> correlates directly 
with the toxic loads of various natural and synthetic combus- 
tibles derived through smoke chemical analyses and laboratory 



animal exposure. It also correlates directly and significantly 
with the main toxic loads (HCI) of three types and various 
kinds of conveyor belts. Although more correlations are 
needed for a variety of combustibles whose main toxic loads 
are other than CO or HCI, the excellent correlation established 
between l/d g n load with smoke toxic loads, the reliability of 
the SPDA, and the simplicity of the methodologies suggest 
their possible use as standards for determining the toxic hazard 
of combustibles during fire. 






77 



REFERENCES 



1. De Rosa, M. I., and C. D. Litton. Oxidative Thermal Degrada- 
tion of PVC-Derived, Fiberglass, Cotton, and Jute Brattices, and 
Other Mine Materials. A Comparison of Toxic Gas and Liquid 
Concentrations and Smoke-Particle Characterization. BuMines RI 
9058, 1986, 14 pp. 

2. . Determining the Relative Toxicity and Smoke Obscuration 

of Combustion Products. Pres. at Symposium on Mining Rescue in 
the Service of Mines Workmen, Bytom, Poland, Sept. 28-30, 1987, 10 
pp.; available from M. I. De Rosa, BuMines, Pittsburgh, PA. 

3. Alarie, Y., and R. C. Anderson. Toxicological Classification of 
Thermal Decomposition Products of Synthetic and Natural Polymers. 
Toxicol, and Appl. Pharmacol, v. 57, 1981, pp. 181-188. 

4. Paciorek, K. L., R. H. Kratzer, J. Kaufman, and J. H. Nakahara. 
Coal Mine Combustion Products — Identification and Analysis Proce- 
dures and Summary (contract H0133004, Ultrasystems Inc.). BuMines 
OFR 109-79, 1978, 140 pp.; NTIS PB 299 559. 

5. De Rosa, M. I. Correlation of Combustion Products From 
Smoldering Conveyor Belts. Pres. at AIHA Conference, Montreal, 
Canada, June 2, 1987, 15 pp., available upon request from M. I. De 
Rosa, BuMines, Pittsburgh, PA. 

6. Hartstein, A. M., and D. R. Forshey. Coal Mine Combustion 
Products: Identification and Analysis. BuMines RI 7872, 1974, 12 pp. 

7. Paciorek, K. L., R. H. Kratzer, J. Kaufman, and J. H. Nakahara. 
Coal Mine Combustion Products — Identification and Analysis. Ultra- 
systems, Inc., Tech. Rep. SN8220-A2, 1974, pp.1-149. 



8. Wagner, J. P. Survey of Toxic Species Evolved in the Pyrolysis of 
Combustion of Polymers. Fire Res. Abs. and Rev., v. 7, 1973, pp. 
1-23. 

9. Barrow, C. S., H. Lucia, and Y. C. Alarie. A Comparison of the 
Acute Inhalation Toxicity of Hydrogen Chloride Versus the Thermal 
Decomposition Products of Polyvinyl Chloride. J. Combust. Toxicol., 
v. 6, 1979, pp. 3-12. 

10. Boettner, E. A. Combustion Products From Incineration of 
Plastics. Div. Res. and Dev., Univ. MI, Final Rep. EPA Contract 
032050, Feb. 1973, 200 pp. 

11. Cullis, C. F, and M. Hirsehler. The Combustion of Organic 
Polymers. Clavendom Press, Oxford, Int. Ser. of Monographs on 
Chemistry, 1981, pp. 12-13. 

12. Paciorek, K. L., R. H. Kratzer, J. H. Nakahara, and D. H. 
Harris. Determination of Products of the Oxidative Thermal Degra- 
dation of Variously Treated Woods and Mine Materials (contract 
J0395008, Ultrasystems). BuMines OFR 4-82, 1980, 182 pp., NTIS 
82-146275. 

13. Levin, B. C, M. Paabo, J. L. Guermean, and F. E. Harrif. 
Effects of Exposure to Single or Multiple Combinations of the 
Predominant Toxic Gases and Low Oxygen Atmospheres Produced in 
Fires. Fund, and Appl. Toxicol., v. 9, 1987, pp. 236-250. 

14. Litton, C. D., L. Graybeal, and M. Hertzberg. A Submicrome- 
ter Particle Detector and Size Analyzer. Rev. Sci. Instrum., v. 50, No. 
7, July 1979, pp. 817-823. 



78 



ELECTROMAGNETIC FIRE WARNING SYSTEM 
FOR UNDERGROUND MINES 

By Kenneth E. Hjelmstad 1 and William H. Pomroy 2 



ABSTRACT 

This Bureau of Mines paper describes a fire warning system for underground mines. The sys- 
tem utilizes the transmission of an electromagnetic signal through mine rock to underground work- 
ings where miners equipped with miniature radio-type receivers are made aware of the presence 
of a mine fire. The microreceiver can be mounted within and powered by a cap lamp battery, or 
mounted on and powered by batteries of mobile equipment. 

Tests of the system at two underground metal mines indicate that the electromagnetic signal 
is capable of penetrating over 762 m of rock with high attenuation characteristics. With the trans- 
mitting antenna located either on the surface or underground, the electromagnetic fire warning 
alarm system has the potential of serving as a fire warning system for many metal and nonmetal 
mines, capable of alerting miners in remote parts of a mine to the existence of a mine fire. 



INTRODUCTION 



The principal safety hazard associated with underground mine 
fires is the rapid spread of smoke and toxic fire gases throughout 
the mine workings. Even miners who are quite far removed from 
the fire itself (up to several miles) can be exposed to life-threaten- 
ing concentrations of these combustion products within minutes. 
This occurs because the mine's ventilation system, which contin- 
uously supplies fresh air to the mine, can circulate the combustion 
products from a mine fire with equal efficiency. In underground 
fires the recommended course of action is to evacuate to the sur- 
face, a refuge station, or other safe location as quickly as possible. 
A fire warning system that is capable of alerting the miners quickly 
would thus save precious time and help insure a successful 
evacuation. 



Evacuation of personnel from underground mine fires can re- 
quire considerable time. A survey of 50 underground noncoal mines 
shows an average evacuation time of 27 min. The range in evacua- 
tion times was from 5 to 85 min and was strongly correlated with 
depth of the shaft. In deeper mines, the length of time is greater 
and therefore can exceed the rated capacity of the presently used 
self-rescuer, which is 60 min operating time in a mine environ- 
ment of 1 pet carbon monoxide. Any delay in warning miners of 
the necessity of donning their self-rescuers can be disastrous (7)? 
thus the benefit of an electromagnetic system capable of sending 
a signal to miners in seconds of time is obvious. 



DEFICIENCIES OF PRESENT MINE FIRE WARNING SYSTEMS 



Given the need for rapid evacuation, it is clear that reliable 
and timely fire warning systems are essential. In typical aboveground 
occupancies (factories, apartment buildings, hospitals, commercial 
buildings, etc.), conventional fire alarms such as bells, gongs, lights, 
whistles, public address announcements, and even word-of-mouth 
are sufficient. However, in underground mines, these methods are 
generally not suitable, and are therefore seldom used. 

Underground mines are characterized by workers who are 
widely scattered over very large areas with little or no means of 



'Geophysicist. 
2 Group supervisor. 
Twin Cities Research Center, Bureau of Mines. Minneapolis, MN. 



communication between groups or individuals. Even a short sep- 
aration between the worker and an audible or visual alarm would 
render the alarm useless, especially if the worker was not in direct 
line of sight of the alarm or the worker was using noisy equipment. 
In many mines, working areas are completely isolated, without links 
to any other part of the mine by telephone, power cable, compressed 
air line, conveyors, rail, or any other continuous or semicontin- 
uous conductor over which a warning signal could be transmitted. 
Most mines are so large that the cost of installing a conventional 
signaling system (bells, lights, etc., and the associated wiring) would 



3 Italic numbers in parentheses refer to items in the list of references at the end of 
this paper. 



79 



be prohibitive. The principal disadvantages of the prior art of 
underground fire warning systems are inherent slowness, vulner- 
ability to damage, and limited mine coverage. 

The most common fire warning system in hard-rock mines 
utilizes the ventilation system to transport the fire warning signal. 
Known as the stench system, it operates by releasing an odorifer- 
ous chemical (the same chemical used to odorize natural gas) into 
the mine's ventilation and/or compressed air streams. When the 
miners detect the odor, they immediately begin to evacuate accord- 
ing to a prearranged evacuation plan (2). The principal disadvan- 
tages of the stench system are the time required for the odor to reach 
the remotest work places, and the tendency for some parts of a mine 
to be consistently missed completely (3). These problems are par- 
ticularly acute in mines with openings having large cross-sectional 
areas, and therefore, extremely slow ventilation velocity. Under 
certain conditions, a fire can even generate its own ventilation forces 
that are counter to the mine's ventilation and further slow down 
or reverse the stench flow. 

The deficiencies of the stench system are well known. How- 
ever because of the lack of a superior alternative, it is still the most 
commonly used system. Considerable research effort has been 
directed toward the development of wireless, radio frequency sig- 
naling systems; however, each has inherent disadvantages that have 
precluded their widespread use in mines. Ultrahigh frequency (UHF) 
systems, because of their negligible through-the-rock transmission 
capabilities, are limited to line-of-sight applications. Once a miner 
travels around a corner such that the pocket pager type receiving 
antenna is not in a direct line of sight with the transmitting antenna, 
the wireless communication link is broken. In order to achieve 
minewide coverage for the warning system, it becomes necessary 



to install transmitting antennas throughout virtually the entire mine. 
In large mines that might comprise several hundred miles of work- 
ings, the cost of such an installation would be prohibitive. 

A second radio frequency system operates in the medium fre- 
quency (MF) spectrum (4). Although it too has limited through- 
the-rock transmission capabilities, it has an advantage over UHF 
systems because specialized transmitting antennas are not required. 
Transmission signals parasitically couple into any continuous or 
semicontinuous metallic conductors present. Thus, a receiver need 
only be within line of sight of any such conductor (power line, rail, 
compressed air pipe, etc.) for the system to operate. The disad- 
vantages of the MF system are that the receiving antenna is quite 
large and cumbersome (worn like a vest with large batteries in the 
pockets), and that many modern mines which utilize diesel -powered 
mobile equipment do not have continuous or semicontinuous metal- 
lic conductors installed throughout the mine. They may be present 
in certain locations, but too many areas would be left unprotected, 
and those miners working in remote parts of the mine may not be 
made aware of the existence of a mine fire. 

In summary, the use of stench in the ventilation system in high- 
back, room-and-pillar mines where air movement is slow can result 
in excessive time delay in sending a fire warning to underground 
miners. A fire warning system using wire for warning signal trans- 
mission can be disabled when rockfalls or explosions break the wire. 
Conventional fire warning systems are usually expensive to install 
in a mine; therefore, the mine company may only install them where 
the majority of miners are working. Those miners working in remote 
parts of the mine may not be close enough to a fire warning device 
and as a result, would fail to be alerted to the existence of a mine fire. 



ULTRALOW FREQUENCY (ULF) ELECTROMAGNETIC FIRE WARNING SYSTEM DESIGN 



The electromagnetic fire warning system described in this paper 
combines the transmission and reception of ULF electromagnetic 
through-the-earth fields as a means of sending a fire warning sig- 
nal to underground miners. The magnetic field generated about the 
energized transmitting antenna together with its accompanying elec- 
tric field is the means of signal transmission. The fields emanate 
from the transmitting antenna in a somewhat spherical manner, 
described by the following equation (5): 



for reception of the through-the-earth signal. The small (15-in-long) 
antenna allows for mobility of the receiver wearer and increases 
the likelihood of survivability of the warning system, since it has 
no long connecting wire that would be exposed to damage from 
fire, rockfalls, or explosions (fig. 1). The transmitting unit is of 
conventional design, without significant size limitation on antenna 
configuration or transmitter power (fig. 2). 



H = 



INAfG], 

2nZ 3 



H 


I 


N 


A 


G 


n 


Z 



magnetic field strength, A/m, 

anntenna current, A, 

number of turns, 

area of the antenna loop, m 2 , 

an attenuation constant, 

the constant 3.14 (pi), 

distance through the transmitting medium, m. 



The transmitter consists of a small signal generator, a 1 ,000-W 
audiofrequency range amplifier, and a transmitting antenna; all 
located either at the surface or underground in the mine. If the mine 
is very deep, the transmitter could be located at a midpoint in the 
mine and the signal then transmitted in all directions. The trans- 
mitting antenna used in tests of the system was made up of 10 turns 
of No. 10 or No. 12 insulated copper wire formed into a 100-ft- 
diam loop. (Line configuration antennas might also be used for 
transmitting.) 

The uniqueness of the system is the mobility of the receiving 
unit, which utilizes a high-permeability wound ferrite core antenna 




Figure 1.— Fire warning system receiver. 



80 




Figure 2.— Fire warning system transmitter. 



Tuning to resonance of both transmitter and receiver antenna 
allows for maximizing transmitting antenna current (and power), 
while maintaining a small-sized receiver antenna for the conven- 
ience of the wearer of the receiver. Tuning of both antennas to a 
common resonant frequency creates a system that discriminates 
against electromagnetic noise, while still accepting the signal in the 
warning frequency range. By choosing frequencies that avoid har- 
monics of power frequency (60 Hz) and using pass band filters in 
the receiver, electromagnetic noise effects can be eliminated. 

The microcircuit receiver is responsive to frequencies from 300 
Hz to 10 kHz. Input to the receiver is from a high-permeability 
ferrite core wound with No. 30 enameled copper wire to form an 
antenna that can be tuned to the carrier wave frequency of the 
transmitting antenna, i.e., 630 or 1,950 Hz. Tuning is accomplished 
by placing capacitance of appropriate size in series with the an- 
tenna to achieve a resonant frequency of choice. The receiver is 
powered by a small battery, therefore it is very portable and con- 
venient to carry. 

For use as a fire warning system for underground mines, the 
receiver would in all probability be powered by the cap lamp bat- 



tery usually worn by an underground miner, or by a vehicle bat- 
tery if mounted on a vehicle. The use of the cap lamp battery as 
a power source for the receiver insures that the receiver would 
always have an adequate power supply since the cap lamp batteries 
are recharged each 24-h period and checked daily. To insure that 
the receiver functions properly, a routine check of it could be made 
at the same time by exposing it to a ULF pulse emitted by a test 
fixture. 

The high-magnetic-permeability ferrite core receiving antenna 
has exceptional magnetic flux gathering capabilities. This makes 
it possible for the antenna to be very sensitive and capable of cap- 
turing even the weakest electromagnetic signal. Through amplifica- 
tion, the generated antenna voltage can be used to initiate a blink- 
ing of the miner's cap lamp in a manner that is recognizable to the 
miner as a fire warning signal. When the miner is certain of the 
warning, he or she can acknowledge the signal by pushing a but- 
ton on the battery to eliminate further blinking of the cap lamp. 

Research has determined that the risk of ULF initiation of elec- 
tric blasting caps is negligible. 






81 



INITIAL FIELD TEST RESULTS 



Initial field tests of the fire warning system were made at a 
local Minneapolis sandstone mine which had 15 m of overburden 
made up of sandstone, limestone, and glacial till. The transmitter 
and a 30-m-diam six-turn loop transmitting antenna were located 
on the surface and the receiver unit was underground. 

For the initial tests, an 8-ohm speaker was used in conjunction 
with the receiver to allow the operator to hear signal reception. The 
first tests were made at low power levels (2 W). The results of these 
tests are shown in figure 3. The area of reception is about 10 times 
the area of the transmitting antenna. The signal was received through 
about 15 m of overburden. The results of this first test were en- 
couraging enough to justify additional field tests of the system. 

A second series of tests was made in the Tower-Soudan 
underground iron mine located in northern Minnesota. Unlike the 
first test, the transmitting antenna was placed underground in a stope 
above the 27th level drift, located 762 m from the main shaft. The 
transmitting antenna was made up of 10 turns of No. 10 copper 
wire formed into a 30-m-diam loop. The receiver (fig. 1) and 
transmitter (fig. 2) were the same as used previously. 

It is possible to establish the voltage generated in a ferrite core 
receiving antenna by measuring across it leads. This was done while 
the receiving unit was placed at the center of the transmitting loop 
antenna. It was established that the ratio of the power in the receiving 
antenna to the power in the transmitting antenna, in decibels, was 
a constant value regardless of the transmitting power levels, pro- 
vided distance between antennas was constant. 

Receiving antenna power levels were then established at various 
points throughout the mine at various transmission power levels. 
The ratio of the power level in the receiving antenna to the power 
level in the transmitting antenna, in decibel loss, is the basis for 
establishing power loss at various distances. The results of these 
measurements are shown in figure 4 and indicate that the through- 
the-rock signal decays with distance in a manner similar to an in- 
verse cubic function. However, computer analysis of the data pro- 
duced a best fit for the data to be a hyperbolic function. The graph 
indicates reception through rock to be in excess of 762 m, the max- 
imum distance of signal reception from the transmitting antenna. 
Maximum transmitter power level was 53.3 W. 

Another in-mine test of the fire warning system was conducted 
at the New Jersey Zinc Co. Sterling Mine near Ogdensburg, NJ. 
A 10-turn 30-m-diam transmitting antenna was positioned at two 
different locations on the surface. At one site, the antenna was posi- 
tioned on overburden of water-saturated saprolite (a claylike mate- 
rial). At the other site, the antenna was positioned on marble (fig. 
5). For this test a 30-cm-diam, 500-turn loop antenna was used to 
measure the magnetic field strength at depth. The field strength at 
depth was compared with the established value of magnetic field 
strength generated by the surface antenna. The ratio of the two 
values of field strength permitted evaluation of attenuation. When 
compared, it was determined that the marble had lower attenua- 
tion characteristics than saprolite and was correspondingly more 
transparent to electromagnetic waves. 

Tests at the Sterling Mine and the two sites in Minnesota were 
done at 2.000-Hz carrier wave frequency. If lower values of fre- 
quency are used, the signal attenuation will be less. Skin depth of 
the signal (the depth at which the signal loses 1/e or 36 pet of its 



original value) is inversely proportional to both frequency and con- 
ductivity as shown by the following equation. 

Skin depth = 6 = (^—) Vi , 

where wja is frequency and a is conductivity. 



Areo of '///'777?7?f7^. 

reception in — r&////////'''' ■ - 
cove indicated jy////////////^ 
by shaded 
area 







3 

I 




!te=y,£=J*=4il=Si3* 



i\SZ^£^iS2= 



Figure 3. — Map showing reception area of first field test. 



14 




1 1 1 


12 




- 


10 


-1 


- 


8 


-\ 


- 


6 




- 


4 


- \ 


- 


2 


ov 


- 




1 1 1 1 1 1 u 



100 200 300 400 500 600 

DISTANCE FROM TRANSMITTER, m 



700 



800 



Figure 4.— Graph of signal attenuation at various distances. 



yj 




Figure 5.— Transmitting antenna on surface. 



If n is assumed to be 4 rr times 10~ 7 H/M for a nonmagnetic 
material, this equation can be reduced to the following: 

503 3 
Skin depth = — ■ 

(fa)V2 

Through further calculations the value of o can be established. 
The two types of overburden at the Sterling mine had values of 



conductivity greater than 10" moh/m, which is a value greater than 
the value of conductivity for most mine rock. A high-conductivity 
material retards penetration by a magnetic field, but since the signal 
was still detectable on the 563-m level, the tests suggest that through- 
the-rock signaling is a viable means of warning miners of the 
presence of a mine fire, and that the system is likely to function 
well in a great many mines. 



CONCLUSIONS 



Successful tests of the electromagnetic fire warning system have 
been completed at two underground metal mines. The maximum 
through-the-rock signal transmission distance measured was 765 
m through rock with high attenuation characteristics. Because most 
metal and nonmetal mines in the United States are less than 914 
m deep, it is believed that the electromagnetic fire warning system 



described has potential for widespread use in the domestic mining 
industry. With improved equipment designs and proper transmit- 
ting antenna placement, it is reasonable to assume that the signal 
could be made to reach miners in remote parts of the mine at great 
distance from the transmitting antenna and serve to alert them to 
the existence of a mine fire. 



REFERENCES 



1. Ontario Provincial Government. Improving Ground Control and Mine 
Rescue; The Report of the Provincial Inquiry Into Ground Control and 
Emergency Preparedness in Ontario Mines. ISBNO-7729-1064-2, 1986, 
108 pp. 

2. Pomroy, W. H., and T. L. Muldoon. Improved Stench Fire Warning 
for Underground Mines. BuMines IC 9016, 1985. 33 pp. 

3. Muldoon. T. L.. T. Lewtas, and T. E. Gore. Upgrade Stench Fire 
Warning System — System Development and Prototype Tests (contract 



H0292002, Foster-Miller Assoc, Inc.). BuMines OFR 136-81, 1981, 142 
pp.; NTIS PB 82-122128. 

4. Stolarczyk, L. G. A Medium Frequency Wireless Communication 
System for Underground Mines (contract H0308004, A.R.F. Products, Inc.). 
BuMines OFR 115-85, 1984, 221 pp.; NTIS PB 86-134103. 

5. Sacks, H. K. Trapped Miner Location and Communication System. 
Ch. in Underground Mine Communication (In Four Parts). 4. Section-to- 
Place Communication. BuMines IC 8745, 1977, pp. 31-43. 



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